Another optional mission that becomes available to us after we complete the Plan B story mission is called Claptraps Secret Stash.
SPOILERS AHEAD!
In order to begin Borderlands 2 Claptraps Secret Stash, we must go and speak to Claptrap in Sanctuary (north side).
As a gratitude for saving him, he agrees to give us his secret stash, but only if we do some simple jobs for him.
Description
Claptrap is willing to grant you access to his secret stash in Sanctuary, but only if youre willing to jump through a few (hundred) hoops for him.
Level: 9
Rewards
Money: $124
Experience: 96 XP
Walkthrough
First of all, we need to speak with Claptrap in Sanctuary to give us the tasks he needs to be done. He will propose us a series of insanely hard tasks, only to change his mind at the end and point us to his stash that is hidden at a very long distance from where hes standing.
Obviously, the stash is right behind us (while facing Claptrap), and its cover will fall apart only if we touch it.
Even though its not what were expecting, it turns out that we can actually share items between our characters if we store them in this stash. Speaking again with Claptrap will allow us to complete the Borderlands 2 Claptraps Secret Stash mission, and we can attend to other missions in Sanctuary.