Large engagements are rarely billed in one go. Milestone (or progress) billing lets you invoice the same accepted quote in stages — 30% on kickoff, 40% at the halfway point, the balance on delivery. Each invoice draws down the line's remaining to bill, so you can never over-bill the order: the figures always reconcile back to what the customer agreed to.

Every billable line carries a remaining to bill figure — the part of that line not yet invoiced. When you raise a milestone invoice you draw an amount against the remaining balance; the line's remaining figure goes down by exactly that much. Once every line is fully drawn, the order is fully billed and there is nothing left to invoice.