On carnivals web site you can either call or order them on line.
I would say your best bet is to call and buy them and tell them
what you want them to do.
I always buy them and turn them in when I get on the ship.
You dont have to mail them in either just take them with you
and turn them on at the purser's desk. they will credit them to
your account there.
Double check when you call but thats how I have done it in the
past and will be doing the same next week.