Add coupon code option to website, integrate with Paypal

My website, [login to view URL], needs the ability to accept coupon codes and gift certificates. I need to give customers a unique redemption code which will credit a defined amount of money toward their orders. For example. they get the code: X01010, and type it into the web site. As a result, $30 is taken off their order before shipping. Each code can only be used once, so I need the script to recognize that. Ideally, as a result of this project I can enter a csv list into my shopping cart to make these codes valid. I am hosted at Microsoft Live Small Business. My shopping cart is just Paypal.