OsCommerce check out issue

I have a client that I built a webstore for through OsCommerce.? There are a few custom features that I would like to implement. After the customer put items in thier cart, and wants to check out, before they check out, I need a drop down list for which department they work in. I also need to create an "access code" box.? I want to create 2 access codes (keep it simple).? One would be for a $25.00 credit, the other would be for a $50 credit. In a perfect world, I would love for the following to happen. Let's say they go in and pick the items they want to purchase.? and it's only $40.00....they put the $50.00 access code in, so they would have a $10.00 credit.? this credit would stay in there account to use at a later time.? they also wouldn't need to go to a paypal check out, because nothing would be charged. I would need to know that so I could bill the corporate office. I can explain in more detail if you need. ## Deliverables I can give ftp access