You would do the booking through one of the interline agencies - Dargul or Perx or others. The cruises are a particularly good deal and the prices that you pay require pre-payment of gratuities as part of the package, so you almost get that "out of the way." For example, a balcony cabin for 3 for a one week cruise for Thanksgiving week last year was about $6600 "retail"; the interline rate for 3 including the 3 gratuities was $2670. And that was for a holiday week and for a balcony.