ATT Trade in scam??

I recently traded in my old galaxy and got a new one with what was supposed to $800 off my new phone over the DPP agreement. I sent my old phone in the white phone protector that came in the box and slapped the only label in the box on it and dropped it off at my local post office. I waited 2 full bills before calling into customer service to see if the credits were just taking awhile to get onto my bill. When I called customer service they stated to me that I would need to submit a case to "back office team" "but that didn't matter because they were going to decline my case anyways" due to a trade in not being initiated at the time of order. I requested that the rep submit the case as I believed the back office team would do right by the customer and apply my $800 promo onto my account after reviewing the issue. He stated that I should be receiving a follow-up with the back office team once a resolution was reached in my case. WRONG. Had to call back 3 days later and get a whole new rep that didn't want to take the time to get a resolution for me and transfered me to the trade in department to get me off his phone. Once at the trade in department, the rep informed me that since it was past the 30 day time frame he is restricted to, he would not be able to assist me. Then, got transferred back over to customer service as they would be the only department to get me a resolution at this point. After further research, the agent was able to confirm that AT&T has my old device but "it went to the wrong warehouse" and "there was no trade in initiated" so I would only be able to get an offer of $350 in a one time credit as a resolution for the issue. Obviously I declined that proposed resolution as it was, frankly, disrespectful. After speaking to an extremely combative and rude manager named Shannon, I was told that "350 was all we could do" . Extremely disappointed with the way Shannon conducted herself I requested to speak to her direct superior. Shannon made sure "to set proper expectations that her manager was going to say the same thing she did". When Lydia came on to the call 30 minutes later, she stated that the most she could do for me was give me a $400 credit towards my account as anything above that amount would "go to back office and be declined" At this point I feel scammed out of my old phone that was paid off and now I am stuck having to take a bill credit for HALF the amount of my original trade in promo. I cannot believe AT&T would conduct business in this way and want to warn all future and current customers about this experience.