I'm in the same boat OP -- just posted this on another thread:
Getting a receipt from a FedEx location is NOT good enough guys.
I got an email from Apple this morning saying there was a problem with my order and to call them. Long story short, I got the trade-in kits for my iPhone 12 and my wife's iPhone 12, packed each box with an iPhone, dropped off at the manned FedEx location across the street, and got a receipt for the two boxes with tracking numbers.
When Apple received the boxes, one was empty, and according to Apple, "showed no signs of tampering." I told them that both boxes had phones in them, and Apple is doing an investigation. The receipt I have is worthless if Apple receives empty boxes from FedEx.
I did my own investigation after my phone call with Apple. Turns out that even though I dropped off both phones at exactly the same day/time (they're both on my FedEx receipt), one was delivered to Apple (near Sacramento) on Monday, October 4th, and the other (I'm guessing the empty box) was delivered to Apple two days later -- on Wednesday, October 6th.
Not sure yet how this is going to turn out, but I do know that in the future I will ALWAYS take my iPhones to my local Apple store and have them do the inspection/trade in.