I don't think you are adding more features to Classic and I don't believe this is in EVO.  How about an option added to AR-B option Prepaid to create an invoice for customers that want an invoice and have it linked to show up in AR-N Customer Deposits.  Just a thought.  Don't know difficult that would be.  Customers tend to get confused when there are to many lines with a positive and negative number on their invoices.