How to write an invoice

The eight things it has to carry

1. The word "Invoice". Not "Statement", which means something else, and not your project's name.
2. A number, unique, from one series that only goes forwards. This is what everything afterwards refers to.
3. Who is billing — your legal or trading name, your address, and your tax or company registration number where your country requires one.
4. Who is being billed — the client's legal entity and its address. Not just the person you talk to: the entity that owes the money.
5. Two dates — the day it was issued and the day it is due. "30 days" is a term; a date is an obligation.
6. The lines — what was delivered, how much of it, at what rate, in what currency. One line per thing somebody might query.
7. The totals — subtotal, then tax and any discount as their own lines, then the amount due.
8. How to pay — the account or address, complete enough to be used without a follow-up email.

The four that get left off

• A due date, rather than terms. "Net 30" makes the client do arithmetic, and they will do it in their favour.
• A purchase-order or reference number, if the client uses them. Without it an invoice can sit unpaid in a system that never matched it to anything.
• What happens if it is late. A sentence in the terms, agreed in advance, is worth more than three polite emails.
• The currency, unambiguously. "$" is at least four currencies. The document's currency code settles it.

Numbering

One series, ascending, no gaps and no repeats. Pick a prefix and never change its shape mid-year: `INV-0001`, or `2026-014` if your accountant prefers the year in front. The number should be taken when the document is issued, not when you open a blank one — otherwise every abandoned draft leaves a hole you will be asked about.

In Perly the series and its prefix are a setting, and the number is reserved at save for exactly this reason.

Describing the work

Write the line for somebody who was not in the meeting. "Design" is not a description; "Landing page — three concepts and two revisions, October" is. If the engagement is hourly, the hours belong on the invoice, not in an attachment. If you are passing a cost through, say what it was and show it at cost.

Tax

Whatever applies to you goes on as its own line at its own rate, never folded into the rate above it, so both sides can see the same two numbers. Whether it applies, at what rate, and what you must print beside it — a VAT number, a reverse-charge note, an exemption — is a question for your accountant and your jurisdiction, not for an invoicing tool.

Then send it and keep the record

Send the PDF, or a link to the document, and keep a copy of what went out — a copy that does not change afterwards. If you correct the client's address next year, last year's invoice should still read as it did the day it was issued. Make one now and the shape above is already on the page.