Issue with phone return

Okay so I'm fed up with AT&T customer service. I've had to chat in several times, and I have yet to get a solution. I bought the new Galaxy S9+ back in March when it came out from Best Buy. I received the return shipping label for my old phone and it said I had 14 days to return. Then I received my new phone and a text message from AT&T saying I had until 4/26 to return the phone, which was more than 14 days, but since it was from AT&T I knew I had longer to return. Well, my computer crashed before I was able to return the phone and I lost the return shipping label. I contacted Best Buy first and they said I would have to contact AT&T to get a return shipping label. I did that, but the representative said to just return the device to a store. So I tried that and then at the store they said no they don't accept returns and I have to do that online. So I contacted customer service again and got someone to send me a return shipping label. I returned my device on 4/23 and assumed that since it was postmarked before 4/26 that would be fine. AT&T sent the device back to me and said that since they didn't get it back by 4/26 they couldn't accept it. So I contacted AT&T again to talk to someone. They said they would notate my account and then for me to send the phone back again and put on the paperwork to see the notations. I did that including the chat transcript and the copy of the receipt where I sent the phone back on 4/23. I contacted AT&T again on Sunday because I knew they had received the device and my account had not gotten credited. They informed me again that because they didn't receive the device by 4/26 there was nothing they could do. First off most of the time when you return something you have until they day it's due back to postmark something. Second the device would have been returned on time had I not lost the return label, which I know was not AT&T's fault, but then when I contacted AT&T the first time they sent me to the store. If they had sent me the return label that time I could have returned the device sooner and it would have arrived on time. I refuse to pay for the old phone. I sent my phone back on time and now they are charging me for it. I need someone to help me out with this. I am very frustrated and I'm ready to leave AT&T. I've never had problems with them before, but this is just ridiculous at this point. Someone please help!!!