I think it's paramount that anyone using Prime Now sets this value to 0% in the app when ordering. Otherwise you're just subsidizing one of the wealthiest companies in the world. If you really want to give the delivery person a tip for whatever reason(they had to walk through mud to get to you) then give them cash. Don't pre-tip in the stupid app that clearly shows amazon "well, we don't need to pay well since clearly our customers are happy to compensate".