Trade in device delivered by USPS but not received by T-Mobile

Did you follow all of the instructions for the return?  Putting the Return Authorization Number in the box as well as on the outside of the box?  In most cases, these devices are not sent to T-Mobile and usually end up at an Assurant facility.  If you don't follow the instructions exactly, they pretty much shaft you.  The last time I sent a device in, I sent it at my expense by UPS, insured, because I know Assurant is one of the shadiest companies out there that the cell providers seem to love dealing with for some reason.  

Try reaching out to the T-Force Team via Facebook or Twitter to see if they can look into things a little more for you.  I hope you have the IMEI of the device, which I would request to be blacklisted if Assurant hasn't received it or says they haven't received it.