Groupon type website pre-paid card feature

Hi. We are looking for a programmer to develop a new feature for our Groupon style website. The feature should act as a prepaid card. When the user is about to buy the coupon, a option is given: pay the total amount by credit card OR buy a pre-paid card of a higher amount for a discounted price. Example, pay the U$20 for the coupon or U$25 for a U$ 30 value pre paid card. When the card is purchased, the money should go to the users "wallet" which already exists, and he uses the money from there to purchase the deal. All the website is on CakePHP.