Why am I being charge a device that I have returned?

Long story short, I had to replace my phone, I received the label and took the device to the office, time passed by and I was not aware that the device was not return yet nor being process at the " at&t warehouse", my bill was charge. By fortune I kept the tracking number, So I called when I got that notice, I was angry, agitated and concern about it, I don't need to keep that broken device, I don't want to let them think I'm trying to do fraud, so that tracking number save me to be charge, I called to at&t to report this, the agent was able to find the tracking number and in deed was still on transit for a few months ago!!! I was recommended to contact the carrier (USPS) to file a claim/complaint so they will take care of the matter. That is a good solution for this matter, keep the tracking number and an eye on the updates, of course I was being responsible to return it...