Returned device but AT&T has not received the package

On 11/23/18 I went to the post office to send an item back to AT&T (a Samsung Galaxy Watch 46 mm), but I did not wait for them to scan them item for it ready to be returned and I also did not notate the tracking number on the smart label that came in the box. Now I’m getting charged for a device that is not in my possession and usps has no record of my return, they keep telling me to contact AT&T. AT&T customer support has no way of helping me, and now I’m stuck. How can I resolve this issue?