I will be doing this in the future (only trading in Apple devices in store). Had a somewhat stressful experience this year trading in an iPhone by mail: sent my iPhone 13 Pro Max which was in pristine condition in a trade-in kit via FedEx after upgrading to iPhone 15 Pro Max, received an email from Apple saying that they received an empty box!!!
Apple asked me to confirm what device I put into the trade-in kit and how I sealed the box. After getting this info from me, they opened an investigation, and resolved the case in my favor (I got the $$ that my trade-in was initially evaluated at).
But that left me with a very bad taste in my mouth, I feel like I cannot trust FedEx to handle Apple trade-ins.
I called Apple to discuss this, and the customer service representative told me that it's totally OK to go to the store and do the trade-in there, even though when you buy a new phone, the only option shown on the web page is "trade-in by mail". She said you just go to the store and they cancel the trade-in by mail and take your old phone or any other device you are trading in.