AT&T Lost my trade in phone

         Ive had a rocky start with my new account with AT&T. Lots of things have been messed up with our upgrade processes and now its looking like I'm going to end up paying for it. First AT&T messed up with my first upgrade when I had to make the purchase over the phone and the customer service rep placed the upgrade order as with no trade in which I had a qualifying phone for trade in which would have made the phone free (one of the main reasons we came to AT&T). But I understand how the paperwork was going to be on that and we decided we'd just keep the phone as a spare and pay the premium. But then with my phone they really messed up. I had a an iPhone under the early upgrade program and had decided to upgrade to the latest phone. the deal was I send back the phone and get credited the remaining total and I get the new phone at a discounted rate. When my phone arrived it had no return shipping stuff for my old phone so I contacted AT&T about it and they said the return shipping would be arriving separately and it should be there soon. so I waited almost two more weeks before I called again saying I had not received my return shipping package. This time the rep said that I should have sent it back in the original shipping box (even though it had no return shipping labels or anything inside. So with all the confusion and me fearing id have to pay the total owed on the phone if not returned soon, I chose to take it into an AT&T store myself. I took it to a corporate AT&T that I've been to before and been helped great. They confirmed that leaving the phone with them was a great solution and they processed the return for me right there and had me sign the paperwork. I made sure to keep the receipt just in case something happened. Well now its been about 3 months and AT&T just contacted me and said they never received my phone and I will be billed the remainder. I immediately went to get my receipt, but found that I had chosen to dispose of it during the holidays while cleaning, thinking that the matter was over. 

        So now what's about to happen is obvious. I'll call saying I gave it to them in person. They'll ask for a receipt and when I don't have one they say "all well" and I get stuck with the bill. This has been the worst experience ever with a phone company and I definitely went through a rough time with Sprint when I was with them. At this rate I'm looking at paying off the bills AT&T has decided to lay on me and then I'll take my phones and leave. I don't see how this issue can be resolved and after being screwed over so much, I can't see me wanting to stay around for more.