Invoice

An invoice is the document a supplier sends a customer to ask for payment for goods or services supplied or agreed. It records who is billing whom, what was supplied, the price, any tax, the total due and the payment terms.

In B2B work the invoice is also evidence. Its date usually starts the payment clock, its number lets both sides refer to it without ambiguity, and its terms are what you point to when payment is late. A vague invoice — no due date, no PO number, no bank details — is one of the most common reasons a payable sits untouched in a customer’s inbox.

What an invoice must contain depends on where you are. In the EU and UK a VAT invoice has legally required fields; in the US the requirements come mostly from the contract and the customer’s accounts-payable process.