When you do not have enough coins to spend on lives, extra moves, or boosters, a pop-up will appear prompting you to purchase more coins. If you use this pop-up to make a purchase, the coins are automatically deducted from the purchase so the item can be added to your account.

You can use the + button next to your coin balance on the top right of the main screen to purchase coins if you do not want any coins to be deducted at the time of the purchase.