Writing an Invoice a Client Cannot Query
Finance teams do not query invoices to be difficult. They query them because something on the invoice does not match something in their system, and until it does, the invoice cannot be approved. Every query costs a week. Here is what stops most of them.
A purchase order number, if they use one
Ask whether a PO is needed before you start. In a lot of companies an invoice without one is not late, it simply does not exist. If they do not use POs, ask what they want in the reference field instead.
Lines that match the quote
If the quote said three days of design and two of build, the invoice should say the same, in the same order, with the same words. Rewriting the description because it reads better is how an invoice ends up on someone's desk to check.
Your own numbering, unbroken
Sequential invoice numbers with no gaps. It matters to your accountant and it tells a client's system that this is a real document rather than a duplicate someone has resent.
Tax stated plainly
Show the rate, the amount and the total separately. If you are not registered for VAT, say nothing about VAT rather than writing zero, which reads as an omission.
Dates that agree
Invoice date, the period the work covers, and the due date. Three dates, and the third one calculated from the first rather than typed in by hand. Most disputes about whether something is late come down to two people reading different dates on the same page.
Get those five right and there is nothing left to query except the work itself, which is a much better conversation to be having.
