Apart from the regular rewards, you get bonus coins for each level you beat. These coins are kept in a safe you can see by tapping the button on the right part of the screen. As soon as you collect 3000 coins, you'll have the chance to buy them at a great price.

The number of bonus coins you get depends on the number of regular coins you get for beating levels: it's multiplied by 10. The more regular coins you win, the more bonus coins you get.

You can either purchase the bonus coins once the required number is reached or continue saving them for later. It's technically impossible to disable the safe feature, however, you're not obliged to buy the coins - it's up for you to decide!