Snacks never run out, but replenish over time. You can only give snacks to customers who ask for them. Customers who want snacks will not order a regular meal until you fulfill their snack order. You can tell if a customer wants a snack by noting the snack icon above their head. 

Desserts, on the other hand, are in limited supply. You can give them to any customer, but some customers will specifically request certain desserts! Be careful about giving away too many desserts early in the level!