As mentioned, you give them a credit card when you check-in. When you check-out all your room charges will be applied to your credit card. You'll get a statement (they'll leave it on your doorknob) the morning you check out.
Also, not all room keys need to have charging privileges. If you have someone in your party who needs to have a key, but not have charging privileges on that key, they can do that at Check-In when they create your keys. You might do this for example where kids might have their own key, but you don't want their keys to have charging privileges (which is also protection in case they lost the key, nobody could start charging on it)
Have a great trip.