I will be bill for a phone I return

My phone screen was cracked,I  did a claim and the tech replace the screen but the camara stopped working, I called again and got a new claim,the tech show and told me that they can't repair it.

I  called again and asurion said the phone will be replace and a third tech visit me with the replacement  and the envelope to return the defective one,all ok at this point,I followed the instructions and mail the the phone.but now Im received a email saying I will charge $380 because asurion didn't received the phone.

I  called and after an1 1/2 hrs

No result att keep asking me for the track number from post office 

The instructions never said to pay for mailing secure.

Att said is asurion,and they just bill,but is obvious that too many phones not been received.

It was just a cracked screen,I was fine with my phone