The Tesla Shop sells normal and premium equipment. It uses a lottery system to dispense both permanent and temporary items. All players share a pool of 200 items in which there is one permanent version, several temporary versions, and mostly random items. When a player uses the Tesla, he/she removes one item from this pool. If that players removes the permanent item, the pool is reset to 200. The amount of items remaining in a pool is visible in the tesla shop menu(X/200). If you are calculating your chances of pulling the permanent out of the pool, simply divide 1 by the remainder of the pool. If there are 100 remaining, your chances are 1%, 50, 2%, etc.