Tendered to Returns Agent Trade-In Credit not received

So, Like everybody else, I returned the device via USPS using the usps label AT&T provided, didnot think much about it since I have used their service before and never had the problem, waited few months since AT&T usually takes few months to process everything and start giving us trade in credit, as i did not got my credit even after few months, reachout to customer support and they said they resolved the issue and will get credit from next month, but when next month came still not credit and talked to support again and again they said they resolved it in their side and i should be getting from next month, but again no credit when bill came. This happened like 3 times and then I called bunch of times talking to few different CS some tried to help , some just didn't knew how can it be helped, and finally one CS told me AT&T hasn't received it yet while some Other CS told me they received it but just need to track where it ended and they needed IMEI no for that. And after searching a lot finally found my USPS tracking no and it is now showing Tendered to Returns Agent. Talked to CS again but didn't help much and no clear answer as how to resolve the issue and how can I receive my credits. 

As I already wasted so much time talking to CS and trying to get the credit, finally got tried and this might be my last resort before posting on BBB. I thing I learned is that when you returned the device for trade-in, best way to do is return the device at store not usps.