If you would like to acquire one of the three bonus buy options, you will need to activate it through the game, by clicking on the ‘BONUS BUY’ button.
The fact that you can do so, however, does not mean that you have to. It is just an option that increases your chances of winning, through either direct entrance to free spins, better odds of winning them, and the likes.
But it’s important to remember that a bonus buy is never going to be a guarantee, no matter how much you pay.
As such, you need to weigh the risk vs. the opportunity each offers, and decide if you have the stomach and budget for any of them.