Hi! This was also not available for me when checking the app or online, so I called NCL and they did it within a few minutes. They only charged for the difference between what I had already paid for (2 extra dinners x 2, shore excursion for pax 2), so you get an immediate discount based on what you've already paid/covered by the PLUS program. We are on a 5-night with 2 ports, and it worked out well for us.