Trade in lost in transit

I have been unable to resolve this concern for three months. I shipped my trade in device in November and the package appears to be lost in transit. Xfinity told me that i would receive the promotional credits but have not given them yet because they haven't received the device. There has to be a procedure in place for situations like this. I did everything as instructed and now I'm stuck paying full price for the device and no longer have the previous device. I've escalated with the post office as well. I wouldn't have traded in my old device if I weren't going to receive any credits.