To get Molly's bag bonus, you must beat levels without losing. For every level you beat, you will earn free boosters to help you at the beginning of a new level. 

The more levels you win in a row, the more boosters you will get at the start of the next level. There are 3 stages of rewards, once you reach the maximum stage, as long as you don't fail a level, you will continue to start levels with the maximum number of boosters. 

When you fail a level, Molly's bag resets and you will have to start from scratch again.