In the previous game, you were generally were able to upgrade your inventory and ammo capacities at specific points in the game.
Helping a claptrap in an area usually rewarded you with a backpack upgrade and you could upgrade your ammo capacities at the nearest ammo dump if you have the money.
However so far in Borderlands 2, I haven't found a claptrap that needed rescuing and the ammo dumps only carry spare ammo. I keep running out of inventory space very quickly and am constantly running out of ammo in heavy firefights which is getting irritating. I have plenty of money, but not enough carrying capacity.
At what points am I able to upgrade my inventory capacity and my ammo capacity?
Best Answer
Ammo and backpack upgrades are acquired on the black market. These purchases have to be made with Eridium; a sought after mineral on Pandora. Once you reach the city of 'Sanctuary', one of the first missions you encounter is entitled 'Plan B'. You will receive 4 Eridium from Scooter, allowing you to make your first purchase at 'Earl's black Market'.
http://borderlands.wikia.com/wiki/Eridium