If you book with Costco as your TA, you receive an 8 - 10 % Costco Cash Card after you return. Some say you can in fact cash it in at Costco. Few TAs can come close to matching. However, DU is getting close if you spend huge bucks upstairs.
http://www.dreamsunlimitedtravel.com/cruise/specials.htm#sbc
This 10 % "credit" is one of the best in the business. Albeit not OBC.
Just an example, which I heard is valid.
I booked onboard [10%] , named Costco [10%] , sailed through Sandy [25%] , and reaped 45% off a cruise, sort of.
Perhaps you could even pay with Target cards and get another 5%.
Forgot to ask Costco. Not sure of the min. payments allowed.
Never did learn the max gift card at Target?
I may have found a new game to keep me occupied.