I personally do not agree with going through a travel agent. Carnival offers the rock bottom best prices and they will beat any price you bring to them.
I have always had great service and I always go through Charlene
Now, that having been said, I suggest the following. Contact the Guest Relations department, 800-929-6400, fax 305-406-4718, email firstname.lastname@example.org
Call, fax and email....get thier attention. I have noticed that the rates fluctaute quite a bit so that may have been the reason for the discrepancie, but still, I think Carnival will be willing to work with you. I had a problem last year, and they were really helpful, but I had to talk with the correct person.
Let me know how it turns out.