gratuity : something given voluntarily or beyond obligation usually for some service; especially : tip
http://www.merriam-webster.com/dictionary/gratuity
Being that it isn't voluntary maybe change the word "TIP" on the reciept to "SERVICE CHARGE".
As for which race tips less or more, I can tell you from experience that it doesn't matter the race. Some people are cheap and some are generous. For me the worst tippers were famous people, mostly musicians. The best tippers are almost always those who work or have worked in a tipped position.