Carnival gives out the same rates to anyone so there are no real coupon codes. Sometimes you will find a TA who has blocked a section of rooms off and they have done so at a time/date that the price may be cheaper than it is at the current date. In this case you may get a cheaper rate and also get some perks.
I would call Carnival and see what their pricing is for the date and ship you would like to sail. I use their online site all the time to book my reservations. However, if you call you may get a room that is not listed online.
Carnival offers early saver rates. It is a rate that you book early and if the price goes down you can fill out an online form and request the price drop. However, the rules on this type of reservation are very strict. If you have to cancel the reservation you loose $50 pp on the room and also you do not get a refund for the remaining amount. Instead you get that credit towards a cruise that must be taken in a specific time period. There are more rules dealing with cats and the such.
Let me say by anyone I mean a TA or other agency is "suppose" to have the same rates as the cruiseline.
Also have seen one time that another online TA had a Cat upgrade for the same price as Carnival had and I went with the TA because Carnival would not give me the cat upgrade. They said that sale had ended and that agency should not be adveritising the price. We did get the cat upgrade with that agency.
I am not a TA, we just cruise a lot.