Three more kinds of invoice, and one you can give up on

A retainer invoice asks for a deposit, a mobilisation advance, a month paid up front. It is owed until it arrives, and once it has, what it collected is applied to the invoices that follow rather than sitting as a second debt beside them.

The applying is the part that matters and the part that was missing: a retainer paid in April is money on the customer, and an invoice raised in June had no way to find it. Any invoice can now apply an advance: every payment with an unspent balance is listed, with what is on account and what the invoice owes.

An advance is not a special record. It is a payment that has not been allocated to anything yet, so a receipt taken before there was an invoice to put it against (the commonest way this actually happens) appears there too, without anybody having called it a retainer.

An invoice nobody is going to pay had two bad options: leave it owing for ever, or reverse it with a credit note. A credit note says the sale did not happen and takes back the tax, possibly on a return already filed.

Writing it off says the opposite. The sale happened, the tax stands, the money is gone: the invoice keeps its total and its place in the books, stops being receivable, and says why on its face. Part of it can be written off when some of the debt is genuinely recoverable, and the whole thing undone when they pay after all.