Yes, you can use them toward onboard credit BUT, the only way I've heard of doing this is by ordering them prior to sailing and noting they are for onboard credit. Luanne Russo has done this and explained this in other threads but I think it goes like this (you can email her to be sure)...
when ordering them through Carnival.com you normally have to put an address where you want the FSdollars shipped. Instead of your address you put "on board credit" on one address line and your cabin number on the other address line. Then they apply it to your cabin rather then mailing them to you.
Yes, they can be used to pay cruise fare. I think the problem is that if you book through a TA then you must pay through that TA. The TA should handle receipt of the FSdollars and sending them to Carnival for payment. Maybe only book with a TA that agrees they can handle this for you.
Life's a Beach ... enjoy it on a Cruise!
Carnival Ecstasy 4-06, Carnival Conquest 9-07
Mariner OTS 9-08, Carnival Conquest 2-09
Carnival Conquest 9-09, Carnival Fantasy 1-10
Carnival Triumph 5-11, Mariner OTS 11-11