You add your credit card when you check-in... They have actually made it easy to add the tip... At the bottom of the bill they print two tip options (18% and 20%) and they give you the exact $ amount for each option... You can check that you want to take the tip option and which $ amount before you give your key card (for the dining plan)... They will take off the dining credit and charge your room the tip at the same time... Really easy.... This way you do not have to carry around an extra card.