Discount Code for Paypal Integration

Hi I'm looking to add a promo code/discount code to integrate with paypal. It's quite simple. There is 1 flat rate price of $105. If someone adds a promo code then they will get the product for $100 at checkout. Ideally the promo codes can be infinite so that they can match to a user: so Fred may be promo code 'yyz', but Peter may be promo code 'abc'. It's in order to create affliates and monitor payments. Many Thanks