The Ultimate Guide to Professional Invoicing

Professional invoicing is the practice of billing clients in a way that is clear, accurate, consistent, and credible - and it is one of the most underrated skills a freelancer, agency, or small business can master. Get it right and you get paid faster, look more trustworthy, and avoid awkward disputes. Get it wrong and you chase late payments, confuse clients, and leak revenue you have already earned. This guide is the complete reference: what to include, how to format, when to send, how to follow up, and how to make the whole process effortless.

Whether you are a solo consultant sending your first bill, an agency managing dozens of monthly retainers, or a bookkeeper standardizing a client's billing, the principles below apply. We will move from fundamentals to advanced practice, with templates, examples, a comparison table, and a checklist you can use today.

What Is Professional Invoicing?

An invoice is a formal request for payment that itemizes the goods or services you provided, states the amount owed, and sets out how and when it should be paid. Professional invoicing is the discipline of doing this well - every time, for every client - so that nothing about your bill creates friction, doubt, or delay.

A casual invoice might be a quick line in an email: "Hi, that'll be $500, thanks." It works, sometimes. But it leaves out the structure that protects you: a unique reference number, a due date, your legal business details, and a clear breakdown of charges. When a client queries the amount three weeks later, or your accountant needs records at tax time, the casual approach falls apart.

The three pillars: clarity, accuracy, consistency

Professional invoicing rests on three pillars.

• Clarity - the client can understand exactly what they are paying for, how much, and by when, without having to ask.
• Accuracy - the numbers add up, the dates are correct, and the details match what was agreed.
• Consistency - every invoice you send looks and behaves the same way, which builds trust and makes your records auditable.

When all three are present, an invoice stops being a chore and becomes a quiet asset: it reinforces your brand, accelerates your cash flow, and keeps your books clean.

Who needs professional invoicing

If you bill anyone for anything, this matters to you - but some groups feel the impact especially hard. Freelancers and consultants live and die by cash flow, so a single slow-paying client can derail a whole month. Agencies juggle many concurrent projects and retainers, where one inconsistent invoice multiplies into dozens of confused clients. Contractors and tradespeople often invoice on completion of physical work, where a clear paper trail prevents "I thought we agreed less" disputes. Creators and online business owners increasingly bill across borders and currencies, where clarity prevents costly errors. And accountants and bookkeepers who standardize a client's invoicing turn a chaotic shoebox of receipts into a clean, auditable system.

The common thread is that none of these people went into business to chase money. Professional invoicing is what lets you spend your energy on the work itself rather than on the awkward, time-draining task of getting paid for it.

Why Professional Invoicing Matters for Getting Paid

Most late payments are not caused by clients refusing to pay. They are caused by avoidable friction - a missing reference, an unclear due date, no easy way to pay, or an invoice that simply got lost in an inbox. Every point of friction is a reason for a client to set your bill aside "for later."

A professional invoice removes those reasons. It tells the client precisely what to do and makes doing it easy. That has a direct effect on your cash flow, which for most small businesses is the single biggest determinant of survival and growth.

There is also a reputational dimension. Your invoice is often the last touchpoint in a project, and it shapes how a client remembers working with you. A polished, accurate, on-brand invoice signals that you run a serious operation. A sloppy one undermines all the good work that came before it.

Finally, professional invoicing protects you legally. A properly structured invoice with clear terms is your evidence if a payment is disputed or you need to pursue an unpaid debt. Vague verbal arrangements are far harder to enforce.

The Anatomy of a Professional Invoice

Every professional invoice contains a predictable set of elements. Memorize these and you will never send an incomplete bill again.

Required and recommended fields

1. Your business details - name, address, contact information, and logo. If you are registered for tax (e.g. VAT in the UK or a sales tax number in the US), include the relevant number.
2. The word "Invoice" - clearly labeled, so it is not mistaken for a quote or statement.
3. A unique invoice number - sequential and never reused (more on this below).
4. Invoice date - the date you issued the document.
5. Client details - the client's legal name, address, and a contact person where relevant.
6. A description of goods or services - itemized line by line, with quantities and unit prices.
7. Subtotal, tax, and total - the math shown transparently so the client can verify it.
8. Payment terms and due date - when payment is expected and what happens if it is late.
9. Accepted payment methods - bank details, a payment link, or card options.
10. A purchase order (PO) number - if your client uses POs, including theirs speeds up approval.

A minimal professional invoice layout

Here is how those fields typically flow on the page, top to bottom:

• Header: your logo and business name, your contact details.
• Below that: the word "Invoice," the invoice number, and the issue date.
• Then: "Bill to" with the client's details.
• The body: a table of line items with descriptions, quantities, rates, and amounts.
• The totals block: subtotal, tax, and the grand total due.
• The footer: payment terms, due date, payment methods, and a short thank-you.

This structure is intuitive for clients because it mirrors what they see from every other professional vendor. Familiarity reduces friction, and reduced friction means faster payment.

Itemizing well

Line items should be specific enough to justify the charge but not so granular they become unreadable. "Website design and development" is too vague; listing forty micro-tasks is overkill. Aim for meaningful units of work - "Homepage design," "Five interior page templates," "Two rounds of revisions" - that a client can recognize and approve at a glance.

For each line, decide whether to bill by quantity and unit price (10 hours at $80) or as a flat fixed fee ($800 for the homepage). Hourly lines reassure clients who want to see effort; fixed lines reassure clients who want price certainty. Mixing both on one invoice is fine, as long as each line clearly states which it is. The test is simple: could the person approving this invoice explain each line to their boss without calling you? If yes, your itemization is doing its job.

Notes, terms, and the footer

The footer of an invoice does quiet but important work. This is where you place a short thank-you, your full payment instructions, and any terms and conditions - late-fee policy, ownership of deliverables until paid, or a reference to your agreed contract. A single line such as "Thank you for your business. Payment is due by 6 July via the link above; a 2% monthly late fee applies thereafter" closes the document warmly while leaving no ambiguity about what happens next.

Invoice Numbering Done Right

A unique, sequential invoice number is not bureaucratic box-ticking - it is the backbone of your financial records. It lets you and your client reference a specific bill unambiguously, prevents duplicates, and is a legal requirement in many jurisdictions for tax purposes.

Common numbering systems

The golden rule is simple: numbers must be unique and never reused or deleted. If you cancel an invoice, issue a credit note rather than reusing the number. Skipping numbers raises questions at audit time, so keep the sequence unbroken.

For a deeper treatment of the strategies, formats, and pitfalls, sequential numbering deserves its own playbook - but the principles above will keep the vast majority of businesses compliant and organized.

Choosing the Right Payment Terms

Payment terms define when you expect to be paid and under what conditions. They are one of the most powerful levers you have over your cash flow, yet many businesses copy a default without thinking about whether it serves them.

Common terms explained

• Due on receipt - payment is expected immediately. Best for one-off, lower-trust transactions.
• Net 7 / Net 14 / Net 15 - payment due within that many days of the invoice date. Common for freelancers and small projects.
• Net 30 - the corporate default in many industries; expect it when billing larger companies.
• Net 60 / Net 90 - long terms favored by big enterprises; tough on small-business cash flow.
• 50% deposit, balance on completion - protects you on larger projects by securing upfront commitment.
• Milestone billing - payment tied to project stages; ideal for long engagements.

How to choose

Shorter terms protect your cash flow but can feel aggressive to large clients with rigid accounts-payable cycles. The right answer depends on your leverage, your relationship, and your own runway. As a rule, default to the shortest term your client will reasonably accept, and always state the terms before you start work - never spring them on the invoice.

Spell out late-payment consequences too. A line such as "A late fee of 2% per month applies to overdue balances" sets expectations and gives you grounds to follow up firmly. In many places you also have a statutory right to charge interest on late commercial payments even without a contract clause.

Matching terms to your situation

Think of payment terms as a negotiation between your cash needs and your client's processes. A bootstrapped freelancer with three weeks of runway cannot afford Net 60; in that case, requiring a deposit and short terms is not aggressive, it is survival. A well-capitalized agency billing a Fortune 500 client may accept Net 30 because the certainty and volume are worth it.

A useful framework is to set a default term for each client tier. For small businesses and individuals, Net 7 or Net 14 with a deposit on anything substantial. For mid-size companies, Net 14 or Net 30. For large enterprises with rigid accounts-payable systems, accept their standard term but protect yourself with milestone billing so you are never carrying the full project value unpaid. Whatever you choose, put it in writing in your proposal or contract so the invoice merely confirms what was already agreed.

Quotes, Estimates, Invoices, and Other Billing Documents

Invoices do not exist in isolation. They sit within a family of billing documents, and using the right one at the right moment is itself a mark of professionalism.

The key documents

• Quote - a fixed, binding price for defined work, valid for a stated period. Use when the scope is clear.
• Estimate - an approximate, non-binding figure for work whose scope may shift. Use when details are still emerging.
• Purchase order (PO) - issued by the buyer to authorize a purchase; you reference it on your invoice.
• Invoice - the formal request for payment once work is done or billing is due.
• Credit note - issued to reduce or cancel a previously invoiced amount, e.g. for a refund or correction.
• Receipt - confirmation that payment has been received; not a request for payment.

Invoice vs receipt - the difference that confuses everyone

An invoice asks for money; a receipt confirms money was received. They are not interchangeable. A client uses your invoice to approve and schedule payment, and your receipt to prove they paid. Sending the right one at the right time avoids confusion and keeps both parties' records straight.

Online vs Paper Invoicing

For decades, invoices were printed and posted. Today the overwhelming majority are digital, and for good reason. Still, it is worth understanding the trade-offs so your choice is deliberate.

Pros and cons

Online invoicing advantages:

• Instant delivery - the client receives it in seconds, not days.
• Built-in payment links so clients can pay in one click.
• Automatic tracking of sent, viewed, and paid status.
• Easy reminders and recurring schedules.
• Searchable digital records for tax and audits.
• Lower cost - no paper, postage, or printing.

Online invoicing drawbacks:

• Requires the client to have email and internet access (rarely an issue today).
• Emails can land in spam if not configured well.

Paper invoicing advantages:

• Familiar to a small number of traditional or offline clients.
• A physical record some industries still prefer.

Paper invoicing drawbacks:

• Slow delivery and slow payment.
• Easily lost or forgotten.
• No tracking, no automation, higher cost.

For all but a handful of edge cases, digital invoicing wins decisively. The combination of speed, payment links, and tracking compresses the time between sending a bill and receiving money - which is exactly the outcome you want.

The hidden cost of paper

The slowness of paper invoicing is easy to see, but its quieter cost is the lack of visibility. With a posted invoice, you have no idea whether it arrived, whether it was opened, or whether it is sitting unread on a desk. You are flying blind until either a payment lands or a deadline passes. Digital invoicing replaces that uncertainty with data: you know the moment a client views the invoice, which removes the "I never received it" excuse and tells you exactly when a polite nudge is worth sending. That visibility, multiplied across every invoice you send, is the difference between guessing about your cash position and knowing it.

How to Send an Invoice the Professional Way

Sending an invoice is more than attaching a PDF and hitting send. The how affects whether you get paid promptly.

A clean sending workflow

1. Send promptly. Bill as soon as work is complete or the billing date arrives. Delay signals that the payment is not urgent - to the client and to yourself.
2. Email the right person. Send to whoever actually processes payments, not just your day-to-day contact. CC your contact so nothing stalls.
3. Write a clear subject line. Something like "Invoice #1042 from [Your Business] - due 6 July" tells the recipient everything up front.
4. Keep the message short. A friendly two-line note, the amount, the due date, and a payment link. Attach a PDF as backup.
5. Include a one-click way to pay. A payment link or portal removes the single biggest source of delay.
6. Confirm receipt. A quick "let me know you received this" prompt reduces the "I never saw it" excuse later.

What good looks like

The best invoices are sent the same business day work wraps, addressed to the payer, and accompanied by a payment link that lets the client settle the bill before they have a chance to close the email. Professionalism here is mostly about removing reasons to delay.

A sample covering email

You do not need to overthink the message that carries your invoice. A reliable template reads: "Hi [Name], thanks again for a great project. Please find attached invoice #1042 for $2,500, due 6 July. You can pay in one click using the link below. Let me know if you have any questions - and please confirm you've received this. Best, [Your Name]." That note is short, friendly, states the amount and due date, and includes a call to action and a request to confirm receipt. It does everything a longer message would, in five lines.

Resist the urge to apologize for sending the bill or to bury the amount in pleasantries. You earned this payment. A confident, courteous tone treats the transaction as the normal, expected close to good work - which is exactly what it is.

Getting Paid Faster: Practical Tactics

Faster payment is the natural reward of professional invoicing. Beyond the fundamentals, a few targeted tactics move the needle.

Tactics that work

• Set up automatic reminders. Polite, scheduled nudges before and after the due date dramatically reduce overdue balances without you lifting a finger.
• Offer multiple payment methods. The easier it is to pay, the faster people pay. Card, bank transfer, and one-click links cover most preferences.
• Take deposits. Securing 25-50% upfront on larger jobs protects your cash flow and filters out non-serious clients.
• Make terms unmissable. State the due date in bold and repeat it in your email.
• Invoice in the client's currency when billing internationally, to avoid conversion confusion.
• Follow up with a system, not your mood. A predictable cadence - reminder at due date, again at +7 days, a firmer note at +14 - keeps things professional and consistent.

A polished, professional invoice with an embedded payment option and automatic reminders does the chasing for you. That is the difference between hoping to get paid and engineering it.

Handling the difficult cases

Not every late payment yields to a polite reminder. When an invoice goes seriously overdue, escalate in measured steps rather than all at once. Start by confirming the client actually received and approved the invoice - a surprising share of "late" payments are simply invoices stuck in an approval queue or sent to the wrong address. Next, send a firm written notice that references your agreed terms and states the outstanding amount and any accrued late fee. If that fails, a formal demand letter, a payment plan offer, or - as a last resort - a small-claims process or a collections service may be warranted.

Throughout, keep records of every invoice, reminder, and reply. That documentation is precisely why professional invoicing matters: a clear, numbered, itemized invoice with agreed terms is far easier to enforce than a vague email. The goal is never confrontation for its own sake; it is to make paying you the path of least resistance at every stage.

A Real-World Example: Maya the Brand Designer

Maya is a freelance brand designer working with small businesses. For her first year she invoiced casually - a number typed into an email, no due date, paid "whenever." She spent hours each month chasing payments and once waited eleven weeks for a $3,200 brand package.

Then she professionalized her invoicing. She built a consistent template with her logo, a sequential numbering scheme starting at 1001, Net 14 terms stated up front in every proposal, and a 50% deposit on projects over $1,500. Each invoice carried a one-click payment link and triggered an automatic reminder three days before the due date.

The result was immediate. Her average time-to-payment dropped from over a month to under a week. Disputes vanished because every charge was itemized and pre-agreed. And because each invoice looked polished and on-brand, clients commented that working with her "felt like working with a bigger studio." Maya did not raise her rates or take on more clients - she simply stopped leaking time and money through weak invoicing.

The lesson is general: the gap between casual and professional invoicing is not about effort per invoice. With the right setup, the professional version is actually less work. It is about building the system once.

Common Invoicing Mistakes to Avoid

Even experienced businesses make these. Each one quietly costs you time, money, or credibility.

The frequent offenders

• No clear due date. "Payment appreciated soon" is not a deadline. Always state an exact date.
• Missing or reused invoice numbers. Breaks your records and raises audit flags.
• Vague line items. "Services rendered - $2,000" invites questions and delays approval.
• Sending to the wrong person. Your project contact rarely controls the bank account.
• Forgetting tax details. Omitting required tax numbers can make an invoice invalid.
• No payment instructions. If the client has to ask how to pay, you have already lost days.
• Inconsistent formatting. A different look every time erodes trust and looks amateurish.
• Waiting too long to send. The longer you wait, the less urgent the payment feels.
• No follow-up plan. Hoping is not a strategy; one polite reminder recovers most late invoices.
• Math errors. A subtotal that doesn't match the total destroys confidence instantly.

Most of these disappear automatically once you use a consistent template or a proper invoicing tool. The mistakes are almost always symptoms of an ad-hoc process, not carelessness.

Why these mistakes cost more than they look

Each of these errors carries a hidden multiplier. A vague line item does not just risk one query - it trains the client to scrutinize every future invoice. A missing due date does not just delay one payment - it sets a precedent that your bills can wait. And an unprofessional invoice does not just look bad once; it quietly recalibrates how seriously the client takes the entire relationship. The reason to fix these is not perfectionism. It is that the cost compounds invisibly over the lifetime of every client account, and the fix - a consistent system - is essentially free once set up.

Invoicing Best Practices

Bring the whole guide together into a repeatable standard you can apply to every bill.

1. Use a consistent, branded template so every invoice looks unmistakably like yours.
2. Number sequentially and never reuse a number; use credit notes for corrections.
3. State payment terms before work begins and restate them on the invoice.
4. Itemize clearly at a meaningful level of detail.
5. Send promptly - ideally the same day work is done.
6. Include a one-click payment method on every invoice.
7. Set up automatic reminders so follow-up is never forgotten.
8. Take deposits on larger projects to protect cash flow.
9. Keep digital records of every invoice, paid or unpaid, for tax and audit.
10. Review your unpaid invoices weekly so nothing slips through the cracks.

Adopt these ten and you will sit comfortably in the top tier of professional billers - paid faster, with cleaner books and happier clients.

Tax, VAT, and Compliance Essentials

Invoices are tax documents, and the rules vary by country. This is not a substitute for advice from your accountant or tax authority, but here are the essentials most small businesses need to get right.

Common compliance points

• Tax registration numbers. If you are VAT-registered in the UK or EU, your invoice must show your VAT number and break out the VAT charged. US businesses include sales tax where applicable.
• Tax point / supply date. Many jurisdictions require the date the goods or services were supplied, which can differ from the invoice date.
• Correct tax rates. Apply the right rate for the product, service, and customer location - cross-border rules can be intricate.
• Record retention. Keep invoices for the period your tax authority requires (often several years).
• International invoicing. When billing abroad, note the currency, any reverse-charge VAT rules, and whether the client is a business or consumer.

Good invoicing software handles much of this automatically: it applies the correct tax, stores records, and produces compliant documents, so you are not manually tracking rules that change.

Invoicing across borders

International invoicing adds a few wrinkles worth flagging. State the currency explicitly on the invoice - "$2,500 USD" rather than just "$2,500" - so there is no ambiguity for a client whose own currency uses a similar symbol. Decide who bears any conversion or transfer fees and say so. For business-to-business sales within the EU, reverse-charge VAT rules may mean you do not charge VAT but must note the customer's VAT number and a reference to the reverse charge. For sales to consumers abroad, different rules apply again. None of this is insurmountable, but it is exactly the kind of detail that should be confirmed with your accountant and then encoded into a template so you never have to think about it twice.

Scaling Your Invoicing with Automation

Doing all of the above by hand is feasible when you send a handful of invoices a month. Past that, manual invoicing becomes a tax on your time and a source of errors. This is where automation earns its place.

What automation handles

• Recurring invoices for retainers and subscriptions, generated and sent on schedule.
• Automatic reminders before and after the due date.
• Online payments via integrated card and bank-transfer links.
• Client portals where customers view, download, and pay their invoices.
• Analytics showing what's outstanding, who pays late, and your true cash position.
• AI generation that turns a plain sentence into a complete, formatted invoice in seconds.

This is the leap modern tools enable. With AI invoicing, you can type something as simple as "Invoice Acme Ltd $2,500 for website development due in 14 days" and get a complete, professional, correctly formatted invoice - number, terms, payment link, and all - without filling in a single field. Aviy built its AI Invoice Generator around exactly this idea: professional invoicing at the speed of a sentence.

Automation does not replace the principles in this guide; it enforces them. Every invoice comes out consistent, numbered, itemized, and on time, because the system bakes the best practices in. You get the credibility of meticulous invoicing without the manual labor.

Summary

Professional invoicing is a learnable, repeatable discipline built on clarity, accuracy, and consistency. A great invoice includes your business and client details, a unique sequential number, itemized charges, transparent totals, clear payment terms with a firm due date, and an easy way to pay. Send it promptly, follow up systematically, and keep clean records for tax.

The payoff is concrete: faster payment, stronger cash flow, fewer disputes, and a brand that signals you mean business. Avoid the common mistakes - vague descriptions, missing due dates, reused numbers, no payment instructions - and adopt the best practices and compliance habits above, and you will be paid more reliably with far less chasing. As your volume grows, let automation and AI carry the load so that professional invoicing becomes effortless rather than a monthly burden. Master this one skill and you protect the most important thing in any business: getting paid for the work you have already done.

Frequently asked questions

What makes an invoice look professional?

A professional invoice has consistent branding, a clear "Invoice" label, a unique number, your business and client details, itemized charges with transparent totals, stated payment terms and a firm due date, and an easy way to pay. Clean formatting and accurate math matter just as much as the content - they signal credibility and remove any reason for the client to hesitate or query the bill.

What information is legally required on an invoice?

Requirements vary by country, but most jurisdictions expect your business name and address, a unique invoice number, the issue date, the client's details, a description of goods or services, and the total due. If you are tax-registered (such as VAT in the UK or sales tax in the US), you must also show the relevant tax number and break out the tax charged. Check your local tax authority for specifics.

How soon should I send an invoice after finishing work?

As soon as possible - ideally the same business day the work is completed or the billing date arrives. Prompt invoicing signals that payment is expected promptly and keeps the work fresh in the client's mind. Every day you delay pushes your payment date further out and quietly signals that the bill is not urgent, which encourages the client to deprioritize it.

What are the most common invoice payment terms?

The most common are "due on receipt," Net 7, Net 14, Net 15, and Net 30, where the number is the days allowed to pay from the invoice date. Larger companies often impose Net 30, Net 60, or Net 90. For bigger projects, deposits or milestone billing are common. Default to the shortest term your client will reasonably accept and always agree terms before starting.

Can I charge interest on a late invoice?

Often, yes. Many countries grant a statutory right to charge interest and recovery costs on overdue commercial payments, even without a contract clause. You can also state your own late fee, such as a percentage per month, in your terms. Always agree these terms up front and reference them on the invoice so the client knows the consequence of paying late.

How do I number my invoices correctly?

Use a unique, sequential number for every invoice and never reuse or delete one. You can use a pure sequence (1001, 1002), a date-based format (2026-06-001), or a client or project prefix (ACME-001). Keep the sequence unbroken - if you cancel an invoice, issue a credit note rather than reusing the number. Sequential numbering keeps records clean and satisfies most tax authorities.

What is the difference between a quote, an estimate, and an invoice?

A quote is a fixed, binding price for clearly defined work, valid for a set period. An estimate is an approximate, non-binding figure for work whose scope may still change. An invoice is the formal request for payment issued once work is delivered or billing is due. Quotes and estimates come before work; the invoice comes at or after delivery.

Should I use online or paper invoicing?

Online invoicing is the better choice for nearly everyone. It delivers instantly, supports one-click payment links, tracks sent/viewed/paid status, automates reminders, and keeps searchable records - all of which speed up payment. Paper invoicing is slow, easily lost, and offers no tracking. Reserve paper only for the rare client who genuinely cannot or will not accept digital documents.

How do I follow up on an unpaid invoice without damaging the relationship?

Use a calm, systematic cadence rather than reacting emotionally. Send a friendly reminder around the due date, a polite nudge about a week later, and a firmer but courteous note after two weeks referencing your stated terms. Keep the tone professional and assume good faith - most late payments are oversights, and a clear, consistent follow-up system recovers the majority without any friction.

Can AI really create a professional invoice for me?

Yes. Modern AI invoicing tools turn a plain sentence like "Invoice Acme Ltd $2,500 for website development due in 14 days" into a complete, formatted invoice - with a number, payment terms, tax, and a payment link - in seconds. The AI enforces best practices automatically, so every invoice comes out consistent, accurate, and professional without manual data entry.

Conclusion

Professional invoicing is not a clerical chore - it is a revenue and reputation lever hiding in plain sight. When your invoices are clear, accurate, consistently formatted, sent promptly, and easy to pay, you get paid faster, dispute less, and present a brand that clients trust. The fundamentals never change: a unique number, itemized charges, firm terms, a real due date, and a one-click way to pay.

Build the system once and it pays you back on every invoice for the life of your business. Adopt the templates, terms, and best practices in this guide, lean on automation as you scale, and professional invoicing will shift from a recurring headache to a quiet, reliable engine for your cash flow.

Related guides

• How to Write a Professional Invoice (Step-by-Step Guide)
• Invoice Numbering Explained: Systems, Rules and Examples
• How to Get Paid Faster With Better Invoices
• Invoice Best Practices for Getting Paid On Time
• Digital Invoicing vs Paper Invoices: Which Is Better?
• Common Invoice Mistakes Businesses Make (and How to Avoid Them)

Sources and further reading

• Invoicing and taking payment from customers (GOV.UK)
• VAT record keeping and invoices (GOV.UK)
• Recordkeeping for small business (IRS)
• Manage your cash flow (U.S. Small Business Administration)
• Invoice (Wikipedia)