I was able to add this to my reservation today by calling them.
in terms of the math, if you are planning to upgrade your drink package anyway (there are many separate threads on that) then the bump for this is only about $15pp per day which is easily covered off by excursions if you plan on those, the dinners and wifi.
For me it works out to an $800 Canadian dollar savings, however that is for a Port intensive trip with only 1 sea day, and I had been planning to do the ncl excursions and wifi anyway.