As far as the Rasul, I'm not sure you can book that online ahead of time. You might have to wait until you are on the ship. You can always ask Disney when you call. Either way, if you are cruising in December, you are still too far out to book. Hope that helps.
You can't book spa treatments (which the rasul qualifies as, I believe) and reservations associated with the REAS package online until either 105, 90 or 75 days out depending on your status (i.e. Concierge/REAS, Castaway Club Member, Everyone else). Also, from what I have read online, you can't book anything online until the cruise is paid in full.
Like Thomas said, either call DCL, or if you used a travel agent call them, to add the REAS package to your cruise.
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