Has AT&T received my trade-in?

I purchased an LG G8 about a month ago through the $300 trade-in promotion. I sent in my prior phone, an LG G6, as that trade-in. I sent the trade in using packaging and a pre-printed label for a return that came with my LG G8, thinking that was the packaging that AT&T wanted me to use to send in my trade-in. A couple of weeks ago, I received a letter in the mail with a pre-paid label asking for the phone that was being traded-in.

How do I find out if my trade-in was received. I would like to make sure I won't be charged incorrectly for the LG G6 and that I will also receive my promotion card from AT&T.