avocado instantly give you a ball when purchased, so if you buy at the end of a level, and only click "next level" after, you will have two of the balls for one round.
also i didnt get a chance to test this but this likely means if you have an avocado in the store and buy it after the level ends but BEFORE clicking a "save them" prompt, you could get a ball for free. (trade in the temporary avocado ball)
edit: also electric ball doesnt seem that good unless you specifically build for it