Is it something that an average person that don't deal with leather stuff everyday may know?
I did not see that question being answered yet. I think the 50usd might be a good deal. But as a business owner, you know your products and the risk of humid and warm areas on this planet. The customer might not be aware of that. Thus, imo, the seller should have a process available for return products. Like a document, with the shipping address and clear instructions on how to pack the returned item to protect it best. Similar to a users manual for the new product. Common sense might be your common sense, but not all are aware of these things. Indeed, you are in the business for almost 10 years or so.
I see it as part of the company's responsibility to help the customer returning the goods in the best way possible. So was there a clear instruction on packaging and how to return the item?