Store credit option?

We sell classes on our website and have frequent drop-outs. Rather than offer refunds, we offer class credits. I've been using discount codes for these, but sometimes they want to use partial amounts of their credit towards a class purchase. Is there any way to provide a credit to a customer where they can make multiple purchases until their credit amount runs out - similar to how a gift certificate works, but I don't want to pay for each person to have a gift certificate 😅