My return shipment delievered to ATT by USP but lost

Hi

I am a customer of ATT

I returned a merchanside due to next upgrade.

The tracking number is ****

It shows as delievered to Dallas Forth Worth 76177

But ATT denies the receivement...

Who is responsible for this?

I need to file a claim...

ATT is charging me for something i am not responsible..

This is between ATT and USPS

Case CM ****

Please verify the tracking number - i should not be penalized!

[Edited for privacy-please do not post personal or unique information such as but not limited to full names, employee ID numbers, email addresses, phone numbers, account numbers, etc.]