I found this because I just had this happen with a watch. ILowry82 this might be too late for you (maybe? Worth trying... hell small claims court might work with this).
I swapped my Asus Zenwatch 1 circa 2013 for a Samsung watch5. I just got the same email about my box being empty. This should help anyone who finds this forum later.
So incase you have this happen to you here is how you resolve it:
1) When you package your phone / watch / etc make sure to photo it going in the box.
2) When you get to FedEx / UPS make sure you get the receipt with a package weight. ALWAYS GET THE RECEIPT!!! If you forget you still have the tracking number as this usually comes in as a PDF you print. The tracking number can get you the receipt and that will have the weight measured at the drop off.
3) Call the customer support like and get a human. No matter if it's Apple / Google / Samsung or whom ever.
4) Let them know the situation calmly. Know the customer support rep probably isn't the person who screwed you.
5) Let them know you have your receipt from FedEx / UPS with the measured package weight. If they are so sure you didn't mail in your return you need a photo of the empty box, and the weight of the empty box where the package is in frame in the scale. You should be able to see the tracking tag in the photo.
Just asking for 5 alone got the support rep to up my call immediately. I didn't need to argue the point anymore. They realized I proved the watch was in the box until I dropped it off at FedEx, and that most likely it was stolen inside the samsung facility. FedEx isn't perfect, but they typically don't have this issue. So they quickly marked my watch as returned and reinstated my $70 credit. If you have this problem this should work no matter the product value. If you have this with a phone return this should work for you as well.
Why this worked
The argument here is that I mailed back an empty box. By having the weight of the package at drop off and asking for them to weigh the packaging material the onus shifts to either FedEx or Samsung. In either case I fulfilled my part of the bargain and Samsung had no right to bill me. They're welcome to chase down FedEx or (more likely) the person in the receiving room who opened the package. They know you've got them dead to rights once you bring up the package weight. They know if they put that empty package on the scale it will be less than what's on the receipt.
From here on out:
I participated because I was trying to avoid e-waste. Honestly after this experience I'm either doing trade ins at physical stores, or doing a person to person sale on ebay. I won't do another mail in ever again. Samsung is definitely on my **** list now. While I like this watch, and it might be the "best in the market" their business isn't honest and it took me calling them out with evidence to get the right result.