Interesting, it seemed from a while (per Heald's blog) that Carnival was veering towards offering this benefit on-line. That didn't seem to be a wise move. Still a benefit for booking on board, Just no buy future OBC, which I have done twice. I guess Carnival doesn't want to carry that liability on the books.
All I ask is a tall ship and a star to steer her by; John Masefield
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, 2014, Caribbean 2013, Coral 2014, Regal 2014, 2016
Star Clippers: Royal Clipper 2015