Another heads up - you can only get extra backpack slots from Claptraps in Playthrough 1. If you do them in Playthrough 2 - even if you didn't get them the first time around, you won't get them.
Luckily, you can jump between the two games. I just hit level 50 and went back to get the 42 backpack slot achievement.
then my game must of have been bugged as I got up to 48 during my second playthrough. Maybe the first 2 clap traps in the instances gave me pack up grades and the rest just gave me items and xp.