Deposit Found System

Hi, I wish a Deposit Found System (virtual money called Point or Coin) where I can accept in my on-line store pre-payment via Paypal from my user in advance so then he/she can start to buy/purchase/download my ebook/audiobook/videobook being sold. A DEPOSIT SYSTEM (view picture), where a buyer can deposit a big amount (like $25, $50, etc.), to buy smaller amounted products (like $1, $5, etc.)? This would be a much needed feature for almost everyone. In my store the product-prices are very low, but, if an item-buyer will have to buy each and every item one by one, that would be a troublesome process. Like, the Envato system. We deposit a large amount in our accounts, and then buy small things without any problem, from that deposited amount. A deposits system (like codecanyon) as when i am selling an item at £0.50, when paypal transaction fee is 3.4% + £0.20, really takes the. I bought this script ([login to view URL])and I want a customisation… add a deposit found sistem with paypal. Cheers.