I booked a cruise last year through www.cruise.com. I highly recommend taking a look at them. They were the cheapest anywhere by far, and a month after booking, I found a price cheaper. I called them up and they discounted my tickets to match and upgraded my room (I had prepaid). I will always book through them.
(A bit of advice...I have heard that a few people had a problem when the same situation happened. If you get a bad rep, hang up and call back to talk to a new person.)
Good luck!
JD in Fla