Adding Credit to Customer

Ok so before I go Wordpress route I need to ask a question about Marketpress. Is there a way I could load a balance to a customer. Let's say they send me $20 through the mail or Paypal and then I go to their account and put that credit in their account. Can this be done. I have a huge customer base that prepays and this is a great plugin, and I am dying to use it, but this is a dealbreaker and I don't want to use prestashop or magento. So is this easy? Is it possible?