Just booked it for upcoming trip on the Dream. Instructions from the chat agent:
To order you can go on our website under My Disney Cruise, In-room Gifts & Amenities, under Onboard Gift and Amenities, Choose a Port, (item SPA RAIN PKG) and download a gift order form and fax it to (407)-566-3760. You can also purchase it by calling 800-601-8455 or 407-566-7000 Monday - Friday 8:00am - 10:00pm (ET) Saturday - Sunday 9:00am - 8:00pm (ET).
I called the 800# and booked 3 days x 2 = 6 passes @ $18.88 per ($16 + 18% gratuity). Multi-day and couples-multi-day are only available when onboard. I'm going to try to negotiate a refund from what I just bought (I know, good luck LOL). But didn't want to risk passes selling out entirely.