We used to have a "prepaid" system at my coffee shop. We had everyone's name and balance in a google spreadsheet and would switch back and forth on the stand to add it all up. We would ring it out as an "Other" payment so we could keep track of them. (we don't take checks so there was no confusion for what the Other payment was for). When they would add money to the account, we had a Prepaid Sold button on square in its own category.

This became quite a hassle as more and more people added cash to the prepaid accounts. When Square started offering Gift Cards, we bought some and switched all the prepaid accounts over to gift cards we keep on file in the store. It saves a lot of time and hassle when we're busy.

Good luck with your coffee shop!