Returned a phone I recently upgraded using USPS but it still has not arrived to the at&t warehouse?

I recently did an upgrade with the "trade-in" promo within the at&t store where I upgraded the iphone 6s to the iphone 12 pro max. I completed the transaction on December 12, 2020 and received the iphone 12 pro max via mail on December 15, 2020. After using the phone for two days I realized that the phone was simply too big and its better to get a smaller phone. I used the same box that the iphone 12 pro max came in and placed the phone back in its box with all the accessories, then attached the "return shipping label" which was provided in the box, and dropped of the box at USPS on December 17, 2020. As of today which is December 30, 2020 the phone has still not arrived to the At&t warehouse and I am worried that this will cross the 14-day return grace period even though I shipped the iphone 12 pro max back after 2 days of receiving it but it is taking more than 2 weeks for at&t warehouse to receive the phone.

Is my return still valid even if it takes more than 2 weeks for the phone to get shipped back despite me shipping the phone back 2 days after receiving it? I also wanted to know how long it takes for a phone to be shipped back and verified for other customers who went through the return process using USPS in case they received a phone they were not happy with or did not suit their expectations?

After reading countless forums online I am worried that the phone wont be received by at&t warehouse on time. I also was able to locate a tracking website known as "pitney-bowes" which a lot of at&t customers recommended checking after dropping off the device at USPS when doing a return, the last update Pitney Bowes tracking website provided was that it has "departed the pitney-bowes facility", I am assuming it will go to the at&t warehouse next but it has been 7 days since that update and there have been no more updates since...Does anyone who has been through a similar scenario know what is going on or what I should do next?

The final question I had was, will I be charged an installment fee for the iphone 12 pro max even though I have shipped the phone back 2 days after receiving it? I do not think it is appropriate to pay for a phone that I am not using let alone have returned two days after using it, I am worried I will be charged for this iphone 12 pro max every single month despite shipping it back? Any help would be greatly appreciated, thank you for taking the time to read this!