This will not work if an invoice was created and the item code "unapplied payments" is used and the invoice is marked as closed since the overpayment was applied to it.  No credits show up under the customer in this case. 

Does QB desktop not have a report that could be run to show you all invoices that have an item code of "unapplied payment" to make clearing them when a new invoice comes in that the finds can be applied to?