I don't know about the sail and sign, but I was on royal caribbean last year and they have a sea pass card. I used my credit card when I was boarding for my sea pass, but on the last day I realized that I had enough cash to cover the amount. I just went to the purser's desk and changed it to cash. It was no problem. I'm sure the policy is probably the same on Carnival. I am going on a Carnival cruise May 07,2006 and will probably do the same thing.
Aventura Spa Palace-04/25/04
Rccl Explorer of Seas-04/17/05