I've never heard of any incentives to use your tax refund (BTW, how did you get a refund
Princess also has a credit card program. You get points for everything you charge and double points for using it for a Princess cruise. Here's the link:
FAQ: Cruise Answer Place : Princess Visa : Princess Cruises
There are also discounts for being active military, having served in the military, being from certain states, being a past passenger, etc.. I recommend that you talk to a good Travel Agent to be sure you get all the discounts you can!