ExStatic Soda costs 3000 points. When purchased, there will be text on the left of the screen saying "ExStatic Ready". When it says this, the next zombie(s) that hit you will get pushed back, dealing damage. This effect only lasts about half a second, and it will take some time to regenerate.
ExStatic is almost the same as ZombieCraft 2, except it costs 500 points and can't be regenerated, so to use it again you need to re-buy it.