Use the flat rate medium box. I would stuff it with both peanuts and wrap the actual iPad with a ton of bubble wrap, unless you have the box. Both are available at Walmart for about $5. Don't forget to insure it though!
Depends on where you are shipping too. There is another Priority box that is very similiar to the picture I am quoting, but it is not flat rate. I shipped mine in it perfectly for half the price of the similar flat rate box.