I ALWAYS select trade-in at time of purchase. The trade in kit is always sent to my house. Yet I always take my trade-in into the store. Apple doesn’t care if you use the trade in kit and ship it to their 3rd party vendor, or physically bring the old device into the store for trade in. THEY DO NOT CARE!
If you do not select trade-in at time of purchase, you run the risk of your trade in value getting lower as each day passes. If you select trade-in at purchase, you lock the trade value in…it’s on the invoice!
Again, Apple doesn’t care if you ship it using the trade-in kit or if you physically bring it in. Regardless, it’s a trade in.