Discount code

I am looking for software or a script that allows me to add a 'discount code' box to a webpage. I need to be able to add the discount codes and percentages in an admin area. When someone enters a code in the box, it changes the prices on the page and the payment links per the information I entered in the admin page. For example, if you enter code 'intro2184' on this page: [login to view URL] It changes the box to show the discount percentage and also changes the pricing and paypal codes at the bottom of the page. I'd like this to be able to be installed on multiple domains - especially on php pages. I'd also like to be able to discount setup fees and monthly fees separately. Thanks.