I had one like this, but not as much off. I trust my PVP completely and if I had a question I would ask.
I THINK I got the discount off the entire amount including add on taxes and fees. Your deal you say is only off the cruisefare. If I had a question, I would call up and have someone explain the discount to me, if you dont have a PVP at Carnival to call just call the toll free number. I always ask until I completely understand, but that is me. My PVP is great and never minds questions.
I would not ask on a message board, I'd just have called up and asked until I understood how they arrived at that price. I would suggest you call and ask. Perhaps they did not give you the $509 price but booked you on a higher deck than you expected to pay for. I would always ask Carnival myself and be sure if as you show, it doesnt seem to add up. CALL AND FIND OUT.
Conquest 7/21/13, Miracle ALASKA 8/27/13, Miracle HAWAII 9/17/13, Celebrity Summit, 11/30/13, Celebrity Summit, 12/7/13, Celebrity Constellation, 2/7/14, NCL Epic 3/2/14, Allure 5/4/14, Allure 11/2/14, Quantum 12/8/14