oscommerce - gift certificate / credit note

I need someone to find and install a module for an oscommerce store that will do the following: I want to be able to create a code (like for a giveaway or something) that can be used by anyone, but each code can only be used once and is worth $x. For example, I decide to give out cards with a code on them, each worth $2 in my store. The code must be something that can't be guessed, and that I can generate at need (if I need 2 today, I just go in and create 2 of them). When the person buys, they can enter the code at checkout and they would get the value as credit before going to paypal. I can also use this to give existing customers in-store credit. I know there are several modules that do this, so please ONLY BID IF YOU ARE FAMILIAR WITH THESE MODULES AND KNOW HOW TO MAKE THEM WORK. It's just a matter of choosing the right module and installing it, so it shouldn't take more than an hour for someone that knows what they're about. Project must be completed by Thursday evening. Payment will be put in GAF Escrow and held for 7 days after completion, to make sure everything workd and keeps working.