If you are making $7.25 an hour serving several dozen people in that hour, $2.50 is a small but meaningful recognition of gratitude. It will not set one up for financial independence but should generate a moment of goodwill between two human beings.

(As well, I'm a Canadian but I believe those are unusual currency denominations so another little plus).

I think for a lot of people, even some on the ever bubbly hacker news, amounts less than $10 are still not considered insulting (context depending).