No one really want your $20 gift. You give $20 for a year effort that mean you thanks him/her effort and he/she will feel more consideration from you.
From 1st grade till now 10th grade. My family always to give teachers gift cards $20-100 twice/year. Also, mail man, truch collectors. It is same that you pay tip at restaurants.
This little money that moves their hearts. If you are in the U.S., you'd better to learn the western style that total different than eastern gift styles.