Sell store credits in OSCommerce 2.2

I have an OSCommerce 2.2 store and wish to add in a 'credit points' feature. This is to allow users to purchase points (1 point = $1; so the customer can buy as many points he wants).Customers are then able to make purchases using these points. Customers need to be able to track their points usage / balance. Each item in the store needs to be assigned a points value. Credit system should be something similar to [login to view URL] The template of the website is this: [login to view URL]