How can I not feel pressured into doing something I don't really want to do?

[NightHawkBuzz]

Yesterday I went to Panda Express and bought food when I was paying for the food I payed with cash and then was owed back change and then the cashier asked me if I would like to "donate the change to a charity"? I said yes but I really wanted to say no because I didn't want to donate the change.  I've been saving up change for myself to eventually cash it in. I felt pressured to do something I didn't want to do out of judgment and criticism. I really wanted to say no but didn't. How can I not be pressured into doing something I don't want to do?