OsCommerce CCGV/Paypal Mod

I have a website running OsCommerce 2.2 with Paypal Pro and the CCGV 5.17 module installed. If an order total is less than the coupon amount, then the credit card should not be charged or even attempted to be charged. Currently customers have to enter a credit card number to advance to the confirmation page even though they don't have to pay any money. Even if they enter a valid credit card number, their credit card is declined because the charge amount is zero dollars or a negative number. I need this fixed. If they have a coupon that will cover the entire order amount, they should not have to enter any credit card info. I need it fixed in two to three days. This is a common problem, I just don't have time to investigate.