Old iphone returned with smartlabe and now it appears to be lost

I returned an old iphone when upgrading to a new one (per At&t requirements). Sent the phone back in a box with smartlabel return, 3 weeks ago. I Have the USPS parcel return service tracking number, (from the smartlabel).  It shows that it was picked up the shipping agent 3 weeks ago and no further tracking information after that. At&t is not showing that they received the phone back and is now charging me for the phone not being returned. However, I did return it using the smartlabel that At&t told me to use. After spending many hours on the phone At&t said they can't credit the charge for the unreturned phone until they get it back. They are passing the buck to the post office. The post office is saying that even tho they had possession of the package it was then picked up by the shipping agent. Therefore I need to take this up with At&t. I did everything I was suppose to do (per At&t instructions...send back using smartlabel returns and note the tracking #. I have had a family plan with At&t for many years, but this is beyond frustrating and making me want to look for another carrier.