Yep, for flat rate that is the cheapest...the boxes come in a couple of bigger sizes for 8.90. Here'a a link to USPS Store, you can order Priority shipping boxes free
http://shop.usps.com/webapp/wcs/stores/servlet/ProductC...0&top_category=11820
Check the buyer's zip code first..sometimes it's much cheaper to ship regular Priority instead of Flat Rate, and if the item is 13 oz or under, First Class is very inexpensive and just about as fast