I don't agree w/Carnival by any means. There could be some sort of a grace period. But that said, (and I mean to sound harsh but...) the balance and payment information is stated in the cruise booking contract. I think in accepting the booking terms, Carnival binds customers to the contract... and assumes each customer reads and understands the terms.
This brings up a few points to think about though...
- It sounds as if you booked this through Carnival directly. It may be worth considering booking through a reputable and experienced TA in the future. TAs can be critical to keeping you notified as to due dates and such. They also monitor for price breaks which may work out in your favor.
- Mark your calendar(s). We have gotten into the habit of putting reminders in our calendars a few days prior to payment due dates. We had a close call once because we neglected to note a due date... and that was our wake up call.
- It may be worth a bit of your time to call Carnival and see if there is anyone who can give you some relief from the higher payment, especially if there were extenuating circumstances. Sometimes you can luck out with an understanding customer service agent... not often but you may get lucky. I caution you to not call when you are angry/upset about this. Blaming the poor schmuck on the other end of the phone isn't going to get you anywhere. It isn't their fault - literally.
Cruise line commercials portray a "carefree perfect world" sort of experience but don't ever forget that this is a highly competitive business and every penny matters because their margins are thin.
I am sorry you had this experience and hope it works out for you. Don't give up on cruising but maybe consider a different line next time.