I think adding a recurring amount to charge, optional to fill out of course, would allow us that use our sites for income to avoid the hassle of creating new callback files in turn we could tie into the current subscription system to say for me add points based on a monthly fee or just a one time fee, currently looking at doing this now to avoid a separate callback, and with a recurring amount we can easily charge $20 for an item with a recurring yearly fee of say $5. That way we don't have to create more than 1 user upgrade per item. Right now I have to have 2 user upgrades, 1 for the initial payment, the second I have to add manually to each customer that is the recurring amount. Pain in the arse to be quite honest.
I'm sure others would approve and go with the same suggestion here.