If you find that the charge is for real, but you didn't charge that additional amount, call the cruiseline and have them double check. I was always under the impression, that your final bill that they slip under you door, was it and has been on all my cruises. I find this so strange and have heard of it before. They can't just add another $400 on, can they? If all else fails, you can always dispute it with your credit card company.