@flipistics has the ones that are usually recommended. Thankfully I have not had that happed in a while!
As an alternative, in the past I have provided funds via a discount on the item they purchased to pay for shipping and they buy postage themselves. They also get a bit of a kick back on the sales tax they paid as well on the refunded amount. I usually kick in a bit extra as a "headache" fee thank you for taking care of my goof up.
...buying them a label is a lot cleaner of course!