You're the asshole here, not them. I have to get delivery for groceries and such because I can't drive for medical reasons. Sometimes I'm really pressed on cash and can't afford more than 10 cents for the tip. You don't know the situation some people live in, and it's childish to go out of your way to lessen the quality of the goods they paid for. Quite frankly, I'd refuse to tip for that behavior alone.