Efficient operations depend on having supplies readily available, which is why it is important for a hiring manager to understand what you might do in this type of situation. Your answer here will reflect upon how proactive you are and how much importance you place on the continued operations of the coffee shop.
Example: "I understand that checking how many supplies are available can keep operations running smoothly and customers happy. If I were in this situation, I would try to locate the needed items in the back inventory first before going to a supervisor or manager. If the supplies were not in the back room, I would try to appropriately substitute where I could and advise customers of the items that are not available."