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A unique invoice number —

sequential and never reused. It is how both sides refer to the document, how you find it later, and what an accountant will ask for first. Gaps in the sequence look like missing records.

Both parties, fully —

your legal or trading name and address, and the client's. "Bill to" should be the entity that owes the money, which is not always the person who hired you.

Invoice date and due date —

both, explicitly. "Net 30" alone forces the client to do arithmetic, and the arithmetic is where the excuse lives.

Itemised lines —

description, quantity, rate and amount. Enough detail that someone who was not in the room can approve it.

Subtotal, tax and total —

shown separately. A client's accounts department frequently needs the tax figure on its own.

Payment instructions —

exactly how to pay — bank details, accepted methods, or a payment link. An invoice without them requires a follow-up email before anything can happen.

Purchase order or reference —

where the client uses them, an invoice without a PO number is often rejected automatically and never reaches a human.

Terms set the deadline, and the deadline sets the behaviour. Research into invoice payment patterns consistently finds shorter stated terms produce faster payment, and that most invoices are paid a similar number of days after the due date whatever that due date is.

Net 30 is the commercial default and what large companies expect. Net 15 or "due on receipt" is normal for small clients and freelancers, and is worth asking for — you are far more likely to get it by stating it than by negotiating it.

A discount for early payment sometimes works: "2/10 Net 30" means 2% off if paid within ten days, otherwise the full amount at thirty. Whether it is worth it is arithmetic — 2% for twenty days early is an annualised cost of about 37%, which is expensive money unless cash flow is genuinely tight.

A late fee clause is worth including even if you rarely enforce it. Its value is mostly in signalling that the deadline is real. Typical is 1% to 1.5% a month, and state law caps what you may charge, so a specific figure should be checked locally.

The other lever is invoicing promptly. An invoice sent the day work finishes is paid meaningfully sooner than one sent at the end of the month, because it enters the client's payment run earlier and because the work is still fresh.

Whether to charge tax is jurisdiction-specific and this is one place where getting it wrong is expensive in both directions.

Most US states do not tax professional services — design, consulting, writing, development — but a growing number tax some of them, and the definitions are narrow and inconsistent. Physical goods are almost always taxable.

The obligation follows nexus: a connection to a state through physical presence, employees, or economic activity above a threshold. Since the 2018 Wayfair decision, economic nexus alone can create a filing obligation in a state you have never visited.

Outside the US the picture differs again. VAT and GST regimes generally tax services, with registration thresholds and reverse-charge rules for cross-border business-to-business work.

If you are not registered to collect tax, do not put a tax line on the invoice. Collecting tax you are not registered for is a real problem, not a rounding error.

Late invoices are normal and mostly not malicious. A sequence that works, and keeps the tone neutral until it should not be:

A few days before the due date, a short reminder that the invoice is coming due. This alone catches the ones that were filed and forgotten.

On the day after, a note that it is now outstanding, with the invoice attached again. Attaching it removes the "can you resend" round trip.

At two weeks, a direct message to your contact asking them to check with accounts payable, since something has usually gone wrong administratively rather than deliberately.

At thirty days past due, a formal notice referencing the terms and any late fee. This is the point at which the tone changes, and it should.

Beyond that: small claims court handles modest amounts without a lawyer in most states, and the threat of it is often more effective than the process. For larger sums, a solicitor's letter costs little relative to the debt.

The best defence is upstream. A deposit before starting, staged payments on longer projects, and a signed agreement stating the terms remove most of this entirely.