If a customer uses the 'Quick Pay' box toward the bottom of their list of unpaid invoices on their dashboard, the system will apply the amount to the oldest invoices - it will just apply to as many of the outstanding invoices as it can (if their payment is more $ than is owed on the single invoice).
If they don't have any more outstanding invoices for the overpayment to apply to then the system will automatically create a credit on their account!
Side note - if they try to overpay on a single invoice (putting in the payment about on a single lone/invoice rather than using the 'Quick Pay' box) then the system will just correct the amount and only process what is owed on the invoice.
