USPS lost my trade in phone needed for a rebate

I have a new question. As i'm reading all these replies, could someone help me? I sent my phone as damaged to assurant and received a new iphone xs. The damaged phone got lost with USPS. I've called tmobile twice now about the lost phone charge and have also been given the run around. I want to know what I can do to not pay this charge for the lost phone as it was not my fault. I've called and filed a claim with USPS as well as assurant but have heard nothing back from them.