Charge for phone I returned (have tracking proof)

We have been long-time ATT customers through IBM discount plan.  In July I upgraded my phone from a 8 plus to an xsmax. I picked up in store but they would not take my return and said I had to ship it. I printed the label that I was sent and dropped it off. Tracking says it was delivered a month ago. ATT says they don't have it scanned in their system and the tracking doesn't mean anything, they say they don't have it, I'm to cough up a $200 charge. I was on the phone hours today and escalated and got the same answer. I cannot for the life of me understand how they can hold the customer responsible when the phone was delivered and they lost it or screwed up. Another guy was supposed to call me back and of course he didn't. Theoretically they could use the same reasoning with everyone returning a device and just say they don't have it, boom, pay us or else.  How can there be no recourse?