An option that should have been added is, 'It depends on the situation'.
I make items for my guildmates, free of charge. If there's a rare involved, they must provide it. I'll eat the cost of the fuel, no biggie.
People outside my guild, I ask them to tip whatever they think is appropriate, and I don't look down my nose at what's offered. It's always enough to cover fuel.
I'd never consider doing it any other way, frankly, and I was uncomfortable not having an option to reflect that because I think most people act the same way or very similar.