Sometimes buyers make an item not as described return when they are not sure what option to pick. It happens, all sellers must be prepared for this & expect it.
But I am not sure I understand the problem you are having.
If you are shipping from the USA to a non USA (international) address, seller is still responsible for buying return postage or at least facilitating the return which would be something like offering the buyer a refund of the amount of the return postage before they send it back.
A very helpful eBay representative told me about "packlink.com" where you can buy international labels from outside your country. I recently used it to buy a customer a return label from a country in Europe back to the USA.
If you are shipping from the USA to an address within the USA, but the buyer's address is really a forwarding service, then they need to also return the item from that address. If this is what happened, and the item got forwarded out of the USA, then the buyer needs to get it back in the USA one way or another.
If you are outside of the USA and you are shipping to a USA address, I can not offer any insight as I do not have first hand experience with such a situation.
What I would recommend to you, and what I always do myself when shipping to a known forwarding service address, is simply to require signature for delivery. Yes, it is a few extra dollars, but it's worth it for the peace of mind, and gives you extra protection in case you get an item not received claim.
(I edited this post to fix a grammatical error & add an extra sentence.)
