How to deal with rejected return FBM

I sell vaporizers for herbal (CBD etc) use. A customer returned an item with the reason “Performance or quality not adequate” with the comment that “The vaporiser is not good enough for vaping tobacco”.

We very clearly stated twice on the listing that this item can not be used for vaping tobacco or nicotine products. Our return policy states “You will be eligible for a full refund on items returned in original condition within 30 days of receipt. You may receive a partial or no refund on used or damaged returns”. We have thus decided that this item was misused. It was sent back stinking of pipe tobacco and we feel this was clearly misuse of the item. It was quite an expensive model and we cannot now resell it, so we have decided to reject the refund and return the item to him.

What I wanted to know is, how can I prove that the item was returned to him? Is there anywhere to log it in the Amazon system, for example? What is best practice?

We are a fair and honest company and rarely reject FBM returns, but we feel we are taking the correct course of action in this case. We just don’t want the buyer claiming that nothing was returned to him, as Amazon (being Amazon) would no doubt side with the buyer, if we don’t have sufficient proof.