OK, what it is - they will have a CD with all the photos on it that you have taken by the ships photographers. You can purchase the CD, the cost varies, depending on the length of the cruise. I think the 7 night one is $349.00. If you purchase the length of cruise CD, you will be able to access/print all the photos on it once at home in whatever size you want. You can pay an additional $50 on this package and get the CD plus all the photos on it printed.
OR
You can purchase a (for example) 20 photo package for $249.00. You will get the CD (which has all the photos on it - in small thumbnails). Shutters will "unlock" the ability for you to access/print 20 photos from the CD at home.
OR
You can purchase a 20 photo package for $249.00 and get 20 printed photos from Shutters.
OR
You can purchase individual photos. Shutters will predetermine sizes (6X8 = character photos; 8X10 formal/pirate/roving photographers). Those will be printed photos.
While the PP reported being able to apply a credit based, I wouldn't count on it. It may have been just an individual employee deciding to do it and not a standard practice.