Yikes, I've been advertizing today as the last day to order for the Christmas deadline and just received the notification for a large print sale a few minutes ago. I went through the checkout process myself (logged out) after reading this thread, and saw the "Your order will arrive in time for Christmas" notification. The shipping was indeed much higher than usual for my small test print. The default shipping is indeed overnight, and if you change it to standard, this message pops up: Christmas Shipping Deadlines: Your order will not arrive in time for Christmas. Please upgrade to overnight shipping or acknowledge below: I acknowledge that this order will not arrive in time for Christmas.
Hopefully these messages were all in place when the person who ordered my print was ordering, as I'd hate to have been misleading...