USPS Lost My Return

I followed all the protocols given to me to return my old iPhone X to get a credit on my bills- I put it in the box and dropped it off at the post office within the deadline I was given.  I have initiated several "chats" with xfinity mobile throughout this time and each time I'm told something different.  The last 2 chats told me their records show it should have been shipped back via FedEx but the e-mail from xfinity support clearly says USPS and I handed the box to a USPS worker who scanned it before I left.  Xfinity chats then tell me it's my responsibility to get a "lost" package letter from USPS since they don't show USPS on their end.  I've tried several times to contact USPS but now that I'm past the 60 day window I can't get anywhere with USPS.   

Because of all of this I'm not receiving a credit on my bill and have been paying more than I should be for the last 3 months.  If I knew this would have been this much of a hassle I wouldn't have upgraded/looked at other mobile providers because I'm getting nowhere and it is very frustrating.