If you buy coins directly through a pop-up offer (for example, when you don’t have enough coins to get extra space, lives, or boosters), part of your purchase is immediately used for that item. The remaining balance is then added to your coin total.

For example, if you choose to use the 900 coins option when you fail a level, and purchase 1000 coins after, once the purchase is completed, an extra space will automatically appear in exchange for 900 coins and you will have 100 coins added to your coin count instead of 1000. 

If you’d like to buy coins without any automatic deductions, you can always visit the Shop from the main screen (tap the coin icon in the top right) and select the package you prefer.