Express gratitude to those who help you, but whom you don't see. When checking into a hotel, people tip the bellman who carried up their bags, but most people do not leave a tip for the chambermaid. Because they rarely meet the chambermaid, they feel no need to leave her anything. But it is good for your character (and obviously good for the maid) to acknowledge the good someone has done you, even if you don't meet the person.

Joseph Telushkin, A Code of Jewish Ethics