866-227-1888 has helped me snag onboard credit more than once. During one sailing, Carnival changed our itinerary because of weather, and someone in a Facebook group said to call and ask about compensation. I did, and sure enough—they added a small onboard credit to my booking. Another time, they matched a promo I hadn’t known about. It’s not guaranteed, but calling honestly gives you the best chance of catching eligible offers you’d never notice online.