Add a promo code to my PayPal Buy Now button

I already have a site taking PayPal payments using the Buy Now button. It is encrypted so that people cannot change the price on their own. All is working well and has been for many years. I now want to add a Promo Code to the purchase so that people can get a 50% discount. I am happy to set the exact price of the product instead of using a percentage discount, if that you be easier. The Promo Code should be entered on the Web site in between the Where did you hear about us and the Buy Now button for ease of use. The Web site I am talking about is: [login to view URL]