USPS lost trade-in phone, cant get $1100 promo credit now?

I followed everything to the tee, took video evidence of the great condition my old iphone 14 pro was in right before I put it in the box and dropped it off at the post office. Have all receipts, tracking numbers, the post office called me and told me they likely lost it and to fill out a lost package search request. I am multiple days past the window Xfinity/Assurant needed to receive the phone to apply the promo credit, even though I shipped it in the middle of the window. I have tried contacting Xfinity mobile customer support 3-4 times over the last two weeks, I have tried calling Assurant (says they're the shipper of record since the return labels are pre-paid for so I cant even file a claim), no one seems to be re-assuring me that I wont be out $1100 promo credit due to the post offices mistake and so I keep paying full price on my monthly bills because "xfinity hasnt received approval from Assurant to apply the promotion yet".

I dont know what more I can do, who to email or talk to, who or where to provide any of my evidence (video, screenshots, voicemails) from the 10+ hours I've devoted to trying and solve this so far... its extremely frustrating. Do other people just file a BBB or FCC complaint to get an executive resolution agent faster? I have fulfilled all of the customer obligations for this trade-in process yet I still dont have any indication of a resolution from any party.