Customers pay the invoice online

The customer taps Pay, pays on a hosted card page, and comes back. The amount defaults to the outstanding balance and is clamped to it, so overpaying is not possible and you are never refunding money you did not want. Charges run on the payment account your business connected — your account, not a platform account sitting between you and the money — and if you have not connected one, there is no button rather than a button that fails.

A customer who can put down a deposit but not the whole draw pays the smaller amount, and the invoice records it as a partial payment with the remainder still owing. The checkout records whether it was a deposit or a full settlement. Materials get ordered on the deposit instead of waiting on a cheque in the post.

On return, the checkout session is verified on the server with your own credentials and checked that it actually names this invoice and this business, and the amount recorded is the one the processor reports — never a figure the browser supplied. It works with nothing configured but an API key, so online payment is not something that starts working only once a webhook has been wired up correctly.

A signed callback from the processor records payments where the customer never came back, and a nightly sweep replays anything settled in the last three days that never reached an invoice. Recording is keyed on the checkout session, so the return trip, the callback and the sweep can all fire and the payment is still recorded once. Money in the bank against an invoice still reading unpaid is the reconciliation this removes.