RCCL I'm pretty sure is notorious for actually enforcing the not-combinable part of their terms. I'm guessing someone over on cruisecritic might also know what the phone number would be, but I don't know that you're going to get real far. I've definitely seen people comment that it's not really worth having the stock for RCCL because you can only use that credit if you don't have any others.

(Carnival Corp does allow for combining the credits. And while it's listed as non-refundable, I've actually ended up being stopped from leaving the ship because I had credit left from a shareholder credit. They actually made me visit the purser's desk to get money back before they'd let me leave the ship.)