Beware! The Carnival redemption option is NOT a carnival gift card. It is a certificate that can only be used towards new bookings. Here is the verbiage from the website:

"Certificates are not accepted for existing reservations and can only be used toward bookings"

I ordered one of these after I booked my Carnival cruise and when I decided that I would be driving and not flying I would would use my WN points towards a Carnival Gift card to use as spending money on board, but that was not allowed! I quickly sent the certificate back and got my points back.

Just keep this in mind.