Largest box available to fit inside Flat Rate Priority Padded Mailer. Not designed as a stand alone shipping box. Simply tape to seal and the box fits snugly in the padded mailers Easy to fold tuck flap technology Made from 100% recycled paperboard, featuring a minimum 97% post consumer waste 9" x 5.5" x 3.5" The Flat Rate Priority Padded Mailer is great for the price, but some of your products need more protection than a padded envelope can provide. You may need a box for that added security. It may cost you as much as $9.62 to send the same items in a Priority Mail Regional Rate Box A.