The main aim of the speaker is to

A lot of people don’t like to give waiters extra money — a tip, but maybe those people don’t understand about waitresses and waiters. You see, we get very low wages, most of the time less than the minimum wage. We count on the tips as part of our salary. If waiters and waitresses didn’t get tips, they wouldn’t get enough money to live.
     (15) People ask me, "What’s a good tip?" I like to get 15% of the bill. So if a customer has to pay $ 20.00 for her dinner, I like to get about $ 3.00 for a tip. Sometimes I expect 20% if I did a lot work for the customer. But do you know something? (16) Very often it’s the person you work the most for who gives you the smallest tips.
    Once I looked up "tipping" in a dictionary. It said that the letters in the word "tip" stand for "To Insure Promptness". No one knows if that is the real meaning of "tip", but it makes sense to me. If we know a regular customer is a good tipper, then we make sure he gets good service. But if someone gives small tips, we aren’t in a hurry to bring him food or get his drinks. (14) So remember, be nice to your waitress and she’ll be nice to you.