No - I think maybe the last poster was referring to the "Future Cruise Certificate" program - where you pay $100 (must be on a credit or debit card - they will not charge your sail and sign account) and receive a certificate that you use as part of your payment towards the next cruise you book.
The cruise must be a NEW booking - you are not supposed to be able to use the certificate to offset a cruise you already have booked, but some have managed to do so.
The certificate you pay $100 for gives you $100 off the cost of your next cruise (break even) plus OBC ranging from $50 for a short cruise to $200 for a long cruise ($50 3-5 days, $75 6 days, $100 7 days, $200 8 days or longer).
If you don't use the certificate, they refund your $100 to the original credit card you purchased it with (thus, the reason they won't charge your sail and sign account nor let you pay cash).
If you actually BOOK onboard, you pay the full deposit for whatever the current promotion is/length of sailing specific. In return, you receive the OBC as detailed above along with 2 vouchers for friends or family to book on the same sailing as you and receive the same OBC. They have (I believe) 30 days from the time you disembark to book their sailings.
Hope this helps.