Hearts (energy) are used to perform different interactions with heroes. Use the surcharge interaction to increase the amount of coins you will earn for your items. Use it on the most expensive items that are available. You can also use hearts to have your customers change their mind about the items they want. Use this to suggest more expensive items to them. Hearts can be obtained from your customers. Compliment them to randomly get one or two hearts, or cause a loss of one heart. Offer them a discount on low cost items such as knives to have a chance to get a heart.