Store credit after return

I'm allowing costumers to return the items bought in case they don't work. How can I give them a credit to use later in the website without refunding them the money?

I had this problem with a client and it seemed impossible for the client to buy a gift card without paying the transaction fee on it. The obvious solution is to pay for the "refund" gift card with a discount, but Squarespace doesn't allow the use of discounts to pay for a gift card.

I've only had to do this once but I did it by giving a personalized discount code for the value of the return. It's awkward because if they use less than the full value, I think I would have to create a NEW discount code for the balance. I wish we could give gift cards instead.