Trade-in phone lost in transit - WHAT CAN I DO?

Hey - anyone gone trough this? I've sent 2 phones (trade-in) at the same time, from the same USPS location (in FL) to the same address Assurant in TX. One of them got there and the other one was never checked-in. Opened a 'research case' through USPS. They called me and closed the case in the same day as they didn't have the item. USPS person from the local office (Person claim to work close to the delivery address where Assurant is located) claimed that there are thousands phone being delivered at that facility and the item was delivered and not scanned. And closed the case. Xfinity agents don't know what to do. There's no number from Assurant. I did not pay the shipping (Xfinity did) so I cannot open a claim on USPS. Bottom line, I did my job and someone along the way just [Edited: "Profanity"] it up. Now, there's no one who knows how to find the phone and I'm here waiting for my $600 credit - and no phone