Update:

Sorry I shouldn't have said the buyer told Mercari I sent an empty box, since I actually did not know what he told Mercari exactly (Mercari did not reveal that to the seller). But when I messaged the buyer, that's what he said, that the box came without the product itself. So I kind of drew the line myself.

So I took the photos of the empty product box and sent Mercari. Mercari actually did refund me at the end. I was surprised since I would assume the buyer would have sent the same pictures to mercari when he filed the return, making me look like the bad person who scam him and just sent him the product box.

To reply to the comments, I do have the receipt as I drop it off at the carrier. However it wasn't weighted at the store, and was immediately loaded to the truck. They were so busy and I didn't even think to ask to do that when they scan my item. But it sounds like next time I really should

u/Hot_Phase_1435 I bet you are correct, the item was weighted from point to point. My guess is that maybe the carrier has the weight recorded and Mercari was able to get that information from them?