Yes Carnival has a room credit for all its cruise lines, including Princess. You must own a 100 shares of Carnival to qualify. The credit cannot be used for the casino or for gratuities. We have used it several times and it is nice to start out with a positive balance, even if it doesn't last that long. If you do much cruising (on Carnival brands) it can be a real plus.
6 day or less = $ 50 room credit
7-13 Days = $100 room credit
14 + days = $250 room credit
Note In England it would be 25 pounds for 6 day or 50 Euros in Europe.