Invoice

It sets out an invoice number, the date, your details and theirs, a line-item description of what was delivered, the amount, any tax, and the payment terms. Large advertisers will not pay without a purchase order number on it. An invoice missing that reference sits in a queue nobody is watching. Thirty-day terms are common and sixty-day terms are not unusual, so the payment date is worth negotiating alongside the fee. Send it the day the deliverable goes live, chase politely once the term date passes, and keep every one, because they are what a tax return is built from.