AT&T charging me for returned iPhones

I'm a post paid customer with the NEXT Every Year payment program. I recently upgraded my 3 iPhones on September 21, 2018 and returned the old phones on September 29, 2018 via USPS. On October 12, I received a text and email from AT&T stating that they did not received the phones. I immediately call AT&T customer service and was told not to worry, they would take care of the issue because the tracking numbers showed that I did return the iPhones on September 29, 2018. On October 29, I received another text stating that they had not received the phones and that I would be charged $458.33 for each phone. On October 30th, I spoke with AT&T four times regarding the issue. One of the agents even insisted that it was my responsibility for USPS to deliver the iPhones to AT&T. I eventually spoke with a supervisor who opened a case and said not to worry... it will be resolved by November 2, 2018, and I would receive a call from AT&T about the resolution.  AT&T never called, so on November 2nd, I called AT&T... no less than ten times, before being transferred to 'Customer Loyalty'. An ICU case was opened and I was told that it would be resolved in a couple of days. Today I called AT&T twice, and spoke to two persons in their 'Customer Loyalty' department. More new cases were opened but the issue is still not resolved, and I was billed $1374.99 for three iPhones that I returned more than a month ago and the tracking information proves that. I've been an AT&T customer for almost 10 years but this incident has me considering other options. After seventeen phone calls regarding this issue, many of which lasted almost one hour each, yet there's no resolution and no one at AT&T seems to care.

Please help.