Hello I am going to tell you how to use Gift Voucher in osCommerce. First install Credit Class & Gift Voucher module.
Creating a gift voucher for a customer to buy is the same as creating a new product with the exception that the gift vouchers model number must start with the word GIFT in upper case. It can be GIFT_25, GIFT_50, GIFT_100 or GIFT25, GIFT50, GIFT100 or whatever you would like to use.
Gift certificate images have been provide in the catalog\images directory in denominations of 25$, 50$ and 100$ values.
You can also create gift vouchers by sending them to your customers via the admin->Vouchers/Coupons->Mail Gift Voucher selection within the admin. To determine what values the gift voucher is used on please see the doc on modules->order total->gift vouchers.
Gift vouchers are a virtual product. In this regard, in many shops, they do not require shipping charges to be added to them. To avert the shipping charge on gift vouchers set the weight of the gift voucher to 0 when creating the product and enable downloads by setting true the value admin->configuration->downloads->enable download.