Also, top ups are possible already in OFN. Just difficult. You can process a payment that is larger than the amount of the order - which is the same as a top up but must be tied to an order.
So I would say this epic has 2 key features:
Enable payment from credit at checkout. This could be a checkbox that shows when someone has credit, or even a voucher code they generate to give a discount off the order equal to their credit (I dont think that solution is a good one, just a brainstorm). It might be enabling 2 payment methods at checkout (overkill).
Enabling a payment to be made that is not attached to an order, such that top ups can be made.
For me 2) has a workaround so is a lower priority. I would agree to starting with part 1 then adding part two after.
But we are getting ahead of ourselves