Minimum Wage Law question (Texas)?

Whenever I have paid with a Debit Card or Charge Card, there is a line where it says tip and you write down the amount of tip that you want to pay. Of course, if the person is using Traveler's Checks, I am not familiar with this method, unless the patron discuss this with the person who takes the Traveler's Check, and just tells them to tip the waiter. All charge slips have a line where you write the tip amount.