USPS lost my trade in phone needed for a rebate

I am facing similar problem now. i upgraded my Phone to Iphone 12 through costco. shipped trade in device on 02Nov2020.  It's still on Transit.  I am calling tmobile every week. first week, tmobile request to wait for week and second day they want me to go to USPS store and check where is  the device. I went to USPS and checked.  USPS requested to file missing mail . I filed missing mail and no further update.  After 2 days, i called tmobile and checked for the trade in credit.  Tmobile representative informed, we will process trade in when we received device.  Today, i called again,  Tmobile Agent responded as "We are filing handset research form". You will get credit in 5 to 7 business days. How i am responsible when i ship trade in device through shipping label generated by T-Mobile ?