I called my closest AT&T corp store on Friday when I got my new phone, but I wasn't about to ask if they would take my trade. I only asked when their slowest time was the following day. I went in at that time, indeed the store was empty, and they took it without issue. I just confirmed via chat that is in their system to give the monthly credits to my account.
Regarding UPS: Did you get a return box from AT&T for your trade? I didn't think they'd be arriving so fast. If you didn't receive the return box, you didn't by chance use the mailing label that was in the new phone's box did you? (I doubt it, but just asking to be sure, as AT&T specifically says not to use that label.)