Yes, the savings certificates are for C&A members.
You can also buy a 'NextCruise' while on a cruise for $100. This is an open booking that will also offer an onboard credit that can vary based on cruise length. When you decide to use it, you pay the remaining amount needed for a deposit and get the OBC from $50 to $200 on the cruise you decided to book. A 7-night cruise gets $100 OBC, longer cruises get the full $200.
You can also transfer the NextCruise bookings to other people, and buy up to 3 of them.