How to handle Ebill Direct Debit Overpayments

We use Ebill to allow Customers to pay for there purchases.  At this time, they select the Invoices to be paid and tender an amount.  If the amount is equal to or less than the Invoices, the payment is handled fine.  We also have Customers that want to pay a set amount, regardless of the amount of Open Items.  This could result in the case where the amount is more than the Open Items, which would need to end up as an On Account payment.  How would this be handled using the Direct Debit process?