I've had guests leave things behind and I mesage to see if they want them back. Sometimes they say, no, just passit on or use as a rag. One woman left a tee-shirt that she did want back, and I just asked the following guest , who lived in the same country as the first, if she could post it when she returned. I gave her $5 for the postage- (it was a tiny package) because the guest who had left it had actually left before her stay was over (nothing to do with my place, it was the first time she'd travelled outside her own country and said she was homesick) and didn't expect or ask for any refund. I even offered, but she declined.
But if a guest expected me to mail something back, I would normally expect them to send me enough $ to cover the postage.