Here is my experience...
LAST YEAR: I traded in an XS iPhone to AT&T in-store (vs mailing it in) for a $800 credit. The rep at the store gave it the OK. And then it sat forever in their backroom, not getting shipped out. Regularly I would get texts reminding me to turn in my XS. Finally, their 3rd party received it but decided it was activation locked, (it wasn't) so they deducted the monthly credits they said they would give me. Spent months trying to follow up and prove with my worthless in-store receipt that it wasn't activation locked. No luck. Can I have it back? No. Finally I gave up.
THIS YEAR: trading in a different phone for the $999 AT&T credits. Bought my phone through Apple so the trade in would be through Apple. They say you can have the phone returned to you if you dont accept the offer. So already a better deal than my AT&T experience last year.
The only source of worry, is that I have the tiniest chip on the bezel of my 11Pro. Screen looks great, back looks flawless, just a super tiny chip on the bezel. Because the Apple condition survey asks any "major" chips or cracks, I said "no" to qualify for the full $999.
It comes time to return the 11Pro by mail, but on Tuesday I decide to try my luck at the Apple Store. They say YES! We'll accept your return instead of mailing it in. Then 2 different managers examine the chip on the bezel and say it will make my credit be deducted to $700. I leave, and take my chances by mailing it in Tuesday afternoon.
Today Thursday I get the email that my trade-in is complete. And they gave me the full $999 credit.
MORAL OF THE STORY:
Turning in the phone "in person" may not be the easiest route after all. For me it was a disaster at AT&T and a disappointment at Apple Store. But in less than 48 hours from mailing it off, I got a full price offer... no headaches!