How about advising them to return the item, send you a copy of the mailing receipt, and you will include the amount in the total refund. After all, they are trusting that you will make the refund, why are they being so careful to make sure they don't get stuck for the return postage? Their request is not a standard business practice as far as I know. I would state that this is your "return policy" and be firm.(unless someone has more experience with this and has a better plan)