Wow, so I just bought Rijn the Specpyre in... Manor of the Damned! because it was less than one dollar with the 25% of coupon. At checkout, entered the code, explicitly deselected all boxes to trade in games I own, made sure to select to use my existing credit.
Then, I check out, and it automatically traded in two of my $0.50 valued games despite that not being necessary since I had enough credit to cover the game.
Seriously, second time GMG checkout has screwed something up. I'm not going to bother contacting them, but just be careful, guys. Even when you choose to NOT trade in your games, it might still glitch.
Funny thing is I almost took a screenshot before clicking "checkout" just to verify I hadn't checked any trade-in boxes.
Each of those games might have been worth $2 later on, so I lose $3 potential credit ($1.50 extra per game on top of $0.50 I got).
I do not trust GMG at all after these two screw ups on their end.