It's just really weird that on my account on the website it displays my credit, but then I can't use it on that very website!
Oh well, thanks for the help!
I don't generally trade in games, but I had a bunch of last-gen stuff to get rid of and about two months ago I brought ~10 games into Gamestop to trade in. I don't use Gamestop for anything else because at this point I buy everything digitally across Steam/PS4/XB1 so I didn't have a PowerUp...