When clients are leaving your office after an appointment or if they're calling in to ask a question, a habit that you may want to get into is checking to see whether they have any Open, outstanding invoices that need to get paid.
The easiest way to do this is to navigate to the client's profile and click on the Payments tab. Under that tab, you'll see the second table down in the Invoices table:
In this table, you can click the "View" button next to the invoice that's listed to navigate to that invoice's detail screen and submit payments or manage the invoice generally:
You could also use the "Delete" button in the list to remove the invoice from your account entirely. Be careful doing this as it will delete all the line items on the invoice as well and the invoice cannot be recovered: