No, we simply make it easy to buy the cheapest USPS postage possible. For most USPS services you can use whatever packaging you like, although some services such as Priority Mail Flat Rate require specific packaging that USPS can provide to you at your local Post Office, or you can order in bulk through USPS.com/supplies
Keep in mind, free USPS boxes that say "Flat Rate" or "Regional Rate" on them require that you buy the corresponding postage. To take advantage of preferred discount services like USPS Priority Mail Cubic, USPS has free non-flat rate boxes you can use: http://support.pirateship.com/en/articles/1831755-free-usps-mailing-boxes-for-priority-mail-cubic
Please reach out to us if you'd like some custom packaging recommendations!