When I originally posted about the "KitKat Mail", I didn't expect to buy one, but my husband brought one home last night. I figured an update mainly to show pictures would be in order.
The back of the box (left side) is for writing a message to your child. You have to put the message on the outside since the box is tightly sealed and nothing can be inserted into it. The front (right side) is for addressing the box. The little box that says "stamp" tells you to use a 140 yen stamp.
Inside the box is a cardboard support and 4 individually-wrapped fingers. As I suspected, the volume of candy is the same as a normal bar (for about double the price of a regular bar).
At select stores during the early sales period, you might be given a free eraser with your purchase. On the back (in Japanese), it tells you that this is not a chocolate KitKat and that it is an eraser. I guess they were worried that you might try to eat the eraser. ;-)