I would pay extra because I don't want the cognitive load of haggling on the cost. When I want/need A ride, it's because I'm either overloaded, away from home, or drunk. These are three times that I didn't want to bother with any more nonsense.
Its not that much. And you can just bypass it by accepting the first offer you get
Having used it in Mexico, they usually have a suggested price that you can just pitch with, and usually people would accept that or add a tiny amount on it.