They have told her that the charges are for the 'credit hold'
They are not charges. These are holds against a credit amount. They don't actually charge anything. They place a hold on an amount against the credit balance of the card. This way a guest can't go buy a diamond ring at port and charge their card to the limit and leave no way for the cruiseline to collect at the end for actual charges. It is done by virtually every cruiseline and any other business that is allowing a credit limit for a period of time before actually running your bill. Most lines hold approx $60. per day from your limit. The only reason for odd amounts is if the limit they are holding is red flagged by their system for actual charges that are nearing or exceding these limits. If they hold $60. a day and you book $500. worth of shore excursions against your onboard account, they'll hold more to be sure you're still able to cover the charges at the end. Same if you're getting $200. a day in casino chips on your onboard account. Their hold is not enough in these cases so they'll increase it. Nothing to worry about with the billing, they are not making any charges to your card without your signature authorizing the exact amount. A credit hold does not require a transaction signature, you did that when you agreed to their terms and conditions to board.