Commendations are the points you receive to purchase upgrades with. These are earned each time you rank up as well as getting Shift codes (see Gearbox Software's website for more info). One way to spend these points is from the menu select "Arsenal Upgrades" and a weapon you have unlocked. Upgrades available for purchase are highlighted blue with a number letting you know how much the upgrade will cost. Normal progression will require Rank 31 to have 30 points to spend and you will have earned a high enough rank to purchase the upgrades needed for the achievement.