Charged for a phone I returned

So when the iPhone X was being released, like many others I preordered it. After receiving my phone, I printed the return label provided and took my phone to USPS to be sent. That was on December 14th. Now I wake up to a message saying I’m being charged $1149  for a phone I shipped back. Before shipping it back I went to the AT&T store to figure out how to return my phone and the woman there told me that the phone should be taken to the Apple store and given to them and that att doesn’t deal with returns AKA it was a half hour before closing and she didn’t want to deal with another costumer because when I looked through my email I found an email that led me to the pre-paid shipping label.  I’ve had problems with AT&T in the past but have stayed with their service, Can honestly say this is the shove that’ll cause me to switch. Anyone else had this problem?