The delivery itself is not paid for when you press order. You're paying for the food and the taxes. It isn't mandatory, but it is expected. Those words mean different things. You're right though, it's still up to you to decide what the tip is.

That's fine though. You believe that people who work low paying jobs do so because they deem themselves to be worth less than you. That says a lot about you. Not tipping because you received poor service is a completely reasonable way of thinking. But refusing to tip when you know the people you are using to make your life easier depend on said tip, means you could give a fuck less about those people, or anyone in the service industry in general