what happens if an xfinity mobile phone trade in isn't received on time?

Recently switches to xfinity and received incorrect information from customer service reps who said I could return my device in store. I went in, and was told I can't submit the device without mailing it in. I was then told by customer service reps that I have to ship it through fedex. After some googling, it looks like it is USPS. Although the label I had said usps, I thought the reps knew what they were saying and that maybe it was being fulfilled by fedex, that was wrong as fedex couldn't scan the barcode. Because of all of this going around in circled and hours on the phone talking to rude reps my phone might not be received by the deadline date. If it's not received on time, will it be immediately returned to me? will it just get lost in the cue and they keep my phone for months or indefinitely? I personally think that Xfinity should rectify this issue since I was repeatedly given wrong information by their customer service but I doubt anything is gonig to happen. I've asked whether I can just cancel this service because I don't even want it anymore with how horrendous their reps are and was told I can't. So I'm stuck with a new phone and don't know what will happen if my old one is received after the deadline. Does anyone know?