Invoicing Systems

An invoicing system has to keep three things straight: what was billed, what has been paid and what is still owed. Payments come in by ACH, card, check and online payment services, sometimes partial, sometimes against the wrong invoice. Each one has to be matched, the balance aged and the right accounts put in front of the person doing collections.

A design we recommend leaves a posted invoice unedited and issues any correction as a credit memo or an adjusting invoice, so the receivables history and the export to accounting still reconcile months later. Payment notifications from gateways are recorded against the provider's transaction or event ID, because gateways can deliver the same notification more than once; Stripe, for example, documents that webhook endpoints might occasionally receive the same event more than once. A duplicate delivery should not record a second payment. Statements, reminder emails and accounting sync are built on top of that record.

If you accept cards, keep card numbers out of the invoicing system by using the processor's hosted payment page or tokens. That can reduce which PCI DSS requirements apply, but the merchant remains responsible for PCI DSS. For the general ledger side, see Accounting Systems.