If you want to leave a higher tip, (We usually did, to reach 20%). Bring some small bills to do so. We rely, on our room charge so much, that a few times, we forgot, to throw a few ones, or fives in our pockets. You cannot just charge a tip, to your room.
If you order things that aren't on the meal plan- Alcohol, spec. drinks mostly for us, you get a separate check, and can add the extra tip for your meal, along w/ the tip for that check on that receipt.