It's just like booking with your own travel agent only they will let many compete for your business. Pick the one with the best price or best perks offered and deal with them going forward. I have booked through them many times without issue.
11 cruises in 2012, 90 nights at sea visiting 15 countries
8 cruises in 2013, 56 nights at sea visiting 7 countries
Next cruise: None scheduled
PM me if you need a cruisemate for any Europe, transatlantic, or Panama Canal cruises
Date, ship, cost pp