Need osCommerce help: discount coupons, PayPal

I have an osCommerce site that has many contributions installed, among them are Discount Coupon Codes and the PayPal IPN. My discounts are not being applied to the total that is sent to PayPal, and I need that fixed. Should be a very short job for a knowledgeable web developer with osCommerce experience. If you don’t indicate experience with osCommerce, don’t expect to be chosen.