Any time I've gotten a call from someone out of state who found my disc, and I wanted the disc back, I've sent them a bubble mailer, with a return address label, stamps, and usually a little legal tender reward enclosed. All they have to do is seal it up, apply the label and stamps, and drop it off at the P.O.
Their obligation ended at making the phone call, so I feel it imperative to make the rest of the return process as easy for those folks as possible.
I've been the Finder in a couple of these scenarios, where the disc owners did the same. Except for the cash tip, which is fine with me. This is the gold standard of lost disc behavior, I think.