My iPad is currently in Hong Kong, and the pencil is in Korea. The iPad is showing a 5/29 delivery and 5/26 for the pencil even though it was shipped a day later.
You're charged when the iPad is preparing for shipment. When I got my computer in February and now it showed preparing for shipment a couple of days before it shipped. My guess is that's when they start working on it. They don't charge you until then because you can cancel the order before that point.
I ordered on the 18th with an estimated delivery of June 2-4, and it shipped on the 23rd.