Everything seemed to go smoothly until I checked my credit card statement about a week later. Best Buy had charged me first for the full price of the phone ($899, I won't mention taxes here), but only credited me $250 after that. Looking in more detail at the trade-in promo, Best Buy splits the $545 value into a $250 "promo" and a $295 trade-in value for the phone. They had only credited me for the promo. I went back to Best Buy and spoke to the Mobile dept, and they thought sometimes it can take 1-2 weeks for the credit to be processed. So I waited another week, went back today, and they still hadn't credited me. Their manager finally had to manually do it, so now I'm whole, but it was a bit of a hassle.
TL;DR: If you do any sort of trade-in at Best Buy, keep a close eye on your invoice and payment statement to make sure you get the full trade-in credit. (This probably applies to any store that accepts trade-ins.)