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Cunard will take a swipe of your Credit Card at ckeck-in and ask you to sign a printed copy authorizing them to debit your onboard account to that card. Dead easy and it takes less than a minute of the check-in time. Have a lovely time and enjoy QM2, I'm sure you will. Richard.
Not sure why you are correct but currently Cunard swipe your card at the check-in point but even if you did give the details on-line any cruise company will still request your card at check-in and swipe it then as well. Perhaps someone else has more details on Cunard policy, might be to do with internet security ?
It should apply to QM2, as well. When you're on board, introduce yourselves to Gary King, the DJ, as he's my husband.