I think you are mixing two things up. When you board the ship the first time, you will be given a card that serves several purposes, including your ship ID when Boarding. They will take your picture to have on a screen when you board to make sure it is you. There is no extra charge for this card.
You will also need another Picture ID for the Port Authorities. They will check it to allow you to get back to the ship. This can be a drivers license, or passport, or other government issued photo ID (military ID, etc.)
You will also need proof of citizenship when you check in. As of the end of this year that proof will need to be a passport, so getting one now wont hurt, as they last 10 years.
Enjoy your trip.