Lost Return Phone

I upgraded my phone and sent my phone back to AT&T in April or early May, checked the tracking for it to arrive, then after a few weeks threw the tracking number away. I didn't think to take a picture, as I'm sure to do next time, and now they are charging us for not returning the phone. I asked on 2 different calls if they have record of the prepaid label they sent me and they couldn't find any. I did what I was supposed to do except keep evidence, I didn't know I'd need! Has anyone had this happen and have it resolved? I don't know what else to do but call back again and ask for a supervisor but don't know what can be done. Please help. I saw there are hundreds of posts with the same problem but don't have all day to search them. Thanks.