I do not receive mail offers from CAS, but I do receive good offers when I call them. You might get a percent off, or you might get a comped cabin (paying admin & port fees.)
I don't know how much you play, or if the tables tracked your play, but it never hurts to call and ask:
For casino information and reservations,
contact a Casinos At Sea Cruise Specialists
9:00AM to 7:00PM EST
Monday to Friday
1-877-PLAY NCL
(1-877-752-9625)