USPS Lost NEXT Return Packages

I upgraded 2 iPhones at Apple Store on 10-17-2015
They were on the Next plan and so I was required to return them to AT&T.

At the time of purchase (Of the new phones at the Apple Store) I received 2 prepaid USPS shipping labels and was instructed to return the old phones via mail.

I received 2 text messages from AT&T in mid November stating that AT&T has not received the returned phone and I would be charged $299.60 for each of the 2 phones.

On 11/17/2015 I contacted AT&T to determine how to resolve the issue of the non-delivered returns. The Customer Service (CS) Rep confirmed that AT&T had not received the returned phones and acknowledged that the two tracking numbers showed the packages were in transit. The CS Rep the informed me that I would need to wait until the two charges of $299.60 show up on the next bill, then dispute them. The CS Rep indicated that she would annotate my account so that the next CS Rep I speak with would have all of the background information when I call back after December 4, the date my next bill would be issued.

On 12/2/2015 I received two charges on my account for $299.60 for non-returned phones.

On December 11, I contacted AT&T to request my account be credited for $299.60 for each of the two returned phones that AT&T indicates were not received. The CS rep asked for USPS tracking numbers that I previously provided AT&T on 11/17/2015. The CS Rep then advised that I would need to work with USPS in locating the two missing packages. I explained that I had already contacted my local USPS facility in and they (USPS) were not successful in locating the packages. The AT&T CS Rep then advised that I would need to contact USPS again and get them to provide a written acknowledgement that the packages were lost.

I explained to the AT&T CS Rep that it's not my responsibility to deal directly with USPS since I am the third party.

I then asked to speak with a manager within AT&T Customer Service. I explained the situation to the manager. He stated that I would need to ask USPS to email me a confirmation that the packages were lost by USPS. I explained to the AT&T manager that since I am the third party, my only responsibility is to ensure that the phone return packages are dropped off and accepted by USPS, and that it should be AT&Ts responsibility to follow up directly with their postal carrier that they contracted. I explained that if I was responsible for purchasing the return postage I would have used a different parcel service and insured the packages.

The AT&T Manager was unwilling to take a customer-friendly position and instead insisted that I, the Customer, spend the time dealing with AT&T's supplier (USPS/Newgistics). I have no leverage with USPS or Newgistics; I anticipate it will be difficult to get cooperation. Also I am traveling through the end of the month so it will be very difficult to find the time to deal with this.

Has it been this difficult for anyone else, or easier?  Has anyone had a similar situation with a positive outcome?