The fine print says if they are unable to fulfill your order, you get a refund. Not much of a guarantee, lol.
So you have to pay for the item right away, and if they don't have enough stock by Christmas, they just refund you or possibly substitute another item.
I thought they might give you a gift card or something if they can't fulfill it, but I guess not. They get your money up front and don't necessarily have to get you the item.