I only tip the providers who charge below 200.
Now, no matters their fees, if I am happy with the service I will go out of my way to promote the guys. When they become regulars, I also like giving little gifts now and then (usually more personal than a gift card), and do little extras for them, like picking them up from their airport or hotel if they are coming to my place.
Most times, they show me their appreciation with lots of extra time, as I rarely pay for more than one hour.