Usually when I buy something I'm trying to get 10% taken off the price.
I have to admit it has never occurred to me to offer 10% above an agreed price and would severely doubt the sanity of anyone who did.
Do you give the Captain 10% of the ticket price too? And the FO? What about the cabin crew who have looked after you so well? Not a bad deal on an airliner with 300 seats! It's gonna be an expensive trip, isn't it?
Do you offer 10% over the till receipt at the supernarket? Or put $60 of gas in the car and pay Mr Shell $65????
Or buy a $25 shirt and pay the man $30?
It's a ludicrous idea, isn't it? Utterly, window-licking bonkers.
So why the digamma would you tip some probably barely trained contract taxi driver?