How to post payments short by pennies to a few dollars in the Enter Cash receipts window.

When we receive a payment from a customer that is pennies or a few dollars short I want to post this to a catch all GL account. I put overpayments of pennie/dollars in this account when entering Cash Receipts, but I can't enter negative amounts. I really don't want to create a credit memo every time this happens. Anyone know how to do this?