I have had a mani/pedi on embarkation day on my last 6 Disney cruises. I have booked them online and in person. I have noticed that they only allow one slot to be booked online and the other two are left open to book in person. (They generally have space for 3 at a time). Last time I booked myself online, when I got on the ship I booked my mom so we could do it at the same time.
Last time I looked the fire and ice pedicure was $70.00 and the fire and ice manicure was $50.00. If you wait until you board, and book 3 different treatments, they will normally have a 10/20/30 special. 10% off the first treatment, 20% off the second, etc...