Adding Coupon and Gift Voucher System to our OSCOmmerce ebooks site

Need to have a coupon system installed on my modified OSCommerce website. (The standard contirbution CCGV is preferred) All of the products on my site are delivered via download, so I need to have it work so that if a customer uses a coupon and the balance is $0.00 or negative, then it takes them directly to the download page for the item rather than the Paypal payment page. (The standard shopping cart for OSCommerce may already do this?)