Returned old phone and still got charged

I upgraded my phone in april. At&t sent a text message on 4/28 that said to return my old phone by 6/1. They also sent a message on 5/10 that said they now had to receive it by 6/1. The link to print the return label would no longer work, so after calling at&t, Best buy, and at&t again and waiting on hold for about 2 hours total, they finally got me a new label on 5/30. The rep I spoke with told me as long as it was shipped by 6/1 it was ok. I printed the label and dropped it at the post office that same day. On my recent bill, I was charged for not returning the device. The tracking number they gave me states it was received on 6/9. When I called, they told me that I did not return the device and they can't find the tracking number I gave them. They also said they will not reverse the charge. I told them that the link they emailed me to track the return states it was delivered to them. They said they can't see it. So now at&t has my old device and is charging me for not returning it. Has anyone had this issue? I've been with at&t over 10 years (since the Cingular days) and have NEVER had an issue with my account. I like the service (even if it is a little more expensive) but the 45 minute phone call I just had seriously has me considering switching carriers.