Verizon charging for not receiving device that we returned

We had a brand new S24 to quit working. We called Asuncion and they created order for replacement. We then received email saying order was canceled due to no comparable replacement available. We called Verizon and they told us to go to Verizon store. We did and the store ordered us new phone to be delivered to the house. We received but also received the one from Asurion( they shipped in error). We called Asurion and they said to send it back with the old device we were returning to Verizon. Verizon provided a postage paid white envelope package that came preaddressed, going to somewhere in TX. We returned both devices in the package via USPS as instructed. Verizon is now charging me for the full price of the new phone because they said they never received old device back. They also said they NEVER use USPS, that it was supposed to go via UPS. The return envelope they provided clearly stated USPS. We were not given any tracking number ,but Verizon is the one who generated the return label. I dont know if the weight of the 2nd phone interfered with the prepaid shipping. We confirmed with Asurion that it was not their return shipping materials. Verizon said they are holding me responsible for the charges unless they receive the device back. We mailed it back 5 weeks ago and I have no way to track. Verizon said they do not have a tracking number for this though they generated the label. I spoke with the PO that I mailed from and they confirmed Verizon DOES use USPS regularly for these white envelope packages. I don't know where to go from here. Has anyone else had a similar experience with Verizon? How did you fix? I'm getting very stressed as the bill is due early May. I keep hoping the phone will arrive there, but it hasn't yet.