Graham here. My Christmas tip is to have a cut off date for Christmas orders.
Everything gets busy before Christmas and the mail can take longer than normal because there are more letters, cards and packages going through the post. I know that most orders get to my customers in 2-3 working days after I take it to the post office, but there can be delays in December.
My cut off date is at least a week before Christmas for items I have in stock. Custom orders need to be in earlier than that so I have plenty of time to make them up, and source paper if I need it.
If orders are local, people can pick them up if necessary, but I make it clear that anything ordered after the cut off date means there is no guarantee the order will arrive in time for Christmas, even with Express Post (and that costs extra).
I've found that having this cut off date can reduce the number of orders that get there late, last year there was one or two that arrived after Christmas day because customers left it too late to order.