Bill the customer and record the payment that closes out the invoice.
The invoice is where the sales cycle turns into real accounts receivable — the point Redovix records that a customer genuinely owes you money, distinct from having merely ordered or received something.
Saving the invoice and receiving payment are deliberately separate, exactly as billing and paying were on the procurement side. An unpaid invoice still counts as a real receivable on your reports — that's the point of splitting it out, rather than only recognizing revenue once cash actually arrives.
Receive Payment is where the loop closes: select the customer, see what's outstanding, and record what came in. A payment can be partial — the invoice stays open for the remainder — or it can cover the full balance, which is what marks it fully paid.