Shipping with Priority Mail Flat Rate is the only time you are required to use a Priority Mail-branded box
Priority Mail Flat Rate service: The only time you have to use a Priority Mail-branded box is if you're shipping a Flat Rate box or envelope. In that case, your postage has to be for the same type of Flat Rate packaging that you used.
For example, you must use a Priority Mail Small Flat Rate Box® if you purchase Priority Mail Small Flat Rate Box® postage.
If you purchase Flat Rate postage on Pirate Ship and you don't use the right corresponding packaging, USPS will charge you after you ship through its Automated Package Verification system.
You can use any box for regular Priority Mail: Outside of Flat Rate, Priority Mail® is just the name of the service class, and you can use any box or packaging that you'd like.
For most cases, it's actually cheaper to use your own packaging instead of a Flat Rate service.
Use our postage rate calculator to find your most cost-effective options.