I am surprised to see that an independent travel agent will get you better prices than a Carnival Personal Vacation Planner. I was thinking I got the best deal thru them.
You may want to read this article, about this subject, that I wrote just a few weeks ago
The Carnival PVP is selling cruises for the price Carnival sells them for. Any agent can get the exact same price. BUT, their commissions are payed out of that price. They can choose to reduce that price by discounting from the commissions.
Carnival does allow you to book with them and then transfer the complete booking to a travel agent. So you can book, and then shop your booking.
In those cases it means less work for the TA, so they should be willing to discount.