For a 7 day cruise for 2, booking usually requires a $500 deposit. Also, $1500 is more than we usually pay for 7 day on Carnival. One way they will try and wiggle out is by taking your money, then later saying that the ship was full or some such. Make sure that you know what portion of your money is going to the actual booking and that it is adequate to account for the deposit amount required by the cruise line. $400 won't do it.
Just be careful. I hope it works out for you.