How To Tip A Tennis Pro During The Holidays

How should I tip a tennis pro during the holidays?

If somebody got me a tennis racket, the grip might not be the right size, this and this, the same with the tennis shoes. You've got to personalize things. So I prefer getting either cash or a card, where I can just go in. Or a gift certificate to a sporting goods store, that would be cool. Because we go through a lot of tennis shoes and clothes. Anything related to tennis. Tennis balls, obviously.