Device Return lost by AT&T or their return service

About 3-4 months after I shipped my phone back in the mail using their box and shipping label, I get an email saying they didn't receive it and are charging my account over $400. I dig through my photos and find a picture of the label and box like they recommended to find the tracking number and find it. When tracking the number via usps it shows is was delivered to a "Returns Agent" and that usps no longer had any information or updates after that. I try contacting AT&T support but they pretty much just say to take it up with USPS. Now I'm dealing with this 3-4 months after I thought everything had gone smoothly after seeing it was delivered to the Returns Agent and AT&T decides to basically scam me and won't resolve ANYTHING. I highly recommend never doing any business with them ever as they are unprofessional, non-helpful, and just overall a bad experience. And even if you do take a picture of the label (like their instructions stated and which I did) there is still nothing they will do about it and just want your money. HORRIBLE!