I have guests leave stuff behind all the time and for me, it's a massive hassle. I am busy all day and packing up the item, getting to the post office, finding a place to park, waiting in line, filling out customs declaration forms and finally paying the cost of shipping - makes me crazy. Before we even go that far, I had to figure out the cost of shipping so that the guest could assess whether it's worth it. THey couldn't even help that much.
Of course there is a cost involved, but the actual cost of shipping is the least of the concerns for me, it's all the other bother around it. Just last week I had a guest leave a plastic cup (like something you'd get at a picnic, not a good plastic drinking glass) FULL of M&M's in the freezer. It was murderously hot outside, so I have to go to the apartment, put the cup in my car, cover it with something so the chocolate doesn't melt, go to the post office and all the other blah blah. The total cost to ship the candy was $17, and it was probably $5 worth of candy. Shipping from USA to Canada.
The guest has still not paid the shipping charges, this was three weeks ago.
I admire hosts who go so far out of their way to return things, I'm moving towards telling the guest they can arrange for picking it up and shipping it. I'll put the item in a box and seal it. That's it.