Lost trade in phone

I traded in my iphone 15 for iphone 17 pro max to receive a $800 discount as their promotion last january 2026. I received the i phone 17 pro max, unfortunately, my i phone 15 didnt get back to its destination. I called usps because i saw that the item was still on its way, but they said because it was about two months, the item must be lost in transit. I asked USPS what to do. They told me to call Xfinity to file a claim for insurance but its only $100 dollars. I called xfinity to file a report about what happened. And the story went on and on. They just making tickets that dont resolve anything. I havent get back my discount and still paying for iphone 17 pro max as full amount. Researched about it, xfinity should be legally responsible for the lost item because they bought the basic USPS Ground Advantage label with only $100 automatic coverage. If ever, they should file the claim to USPS. But every time i call xfinity about the matter, they couldnt help me at all and just making tickets.