Auction script modification

I am about to purchase either YSP Auction script or PHP Pro Bid auction script (see full demo at [[login to view URL]][1]) and need to modify how seller's auction listing fees are paid. Currently, seller can either “pay as you go?? or I can use accounting mode like eBay does and bill sellers. I don't want to use either; I want sellers to buy pre-paid “credits?? then use them to pay listing fees. I need to have a place in admin where I can set different “packages?? of credits (for example, you can buy 10 credits for $10 or 20 credits for $20 and so on - similar to the way a penny auction works.) Sellers will then have a “bank?? of credits assigned to their account to pay for listing fees.