This is certainly a frustrating circumstance. We have faced similar experiences in the past month. I found it useful to identify the product you received on Amazon in error. Show them the URL. Then, describe the difference between the two products: "One is made from wood, while ours is made from plastic. "The dimensions are..... while our product measures...." Offer a small explanation as to how the customer may have inadvertently returned the wrong thing. Provide receipts or lack of, records of communication. Show tracking information, return label details, or some kind of transit confirmation. Clearly state the best resolution for you, usually a partial refund for the lost item that you can no longer sell as "new" (duh). I'll even follow up with a phone call, but do not give the person who picks up your details; just report the ASIN and the issue you're facing, and ask to be transferred. It's an endless loop, or chain of command, that's why many people eat the cost of the claim. It depends on how much time you're willing to invest.