When am I able to upgrade my inventory and ammo capacity?

Question

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?

Answer 1

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

Answer 2

You can upgrade through Crazy Earl with Eriduim. I am currently maxed out at a 27 space backpack. The upgrade costs are set at 4, 8, 12, 16, and 20.