Trade in, Lost???

I am beyond frustrated with Xfinity!!! I did a trade in for my very expensive Samsung phone to what I now have an iPhone and Xfinity is saying that they never received my phone. I have proof from USPS that it was delivered within the 21 day time frame and I have yet to receive a credit for the trade in. This is ridiculous. Xfinity should be doing their part in honoring their contracts with their customers and if Assurant is not doing their part then you Xfinity should handle it with them, the customers should not being going after Assurant if the deal was not made with them! If I don’t hear back from Xfinity soon I will be contacting BBB and also seek legal advice.