Hi, This is from the usps.com website. If any Express or Priority Mail packaging or packaging supplies e.g., Priority Mail tape to seal a Parcel Post® package, are used on any other class of mail, the applicable Express or Priority Mail price will be charged. Misuse may be a violation of federal law. Turning the packaging inside out to conceal the Priority Mail / Express Mail insignia is specific misuse that is not allowed. I have heard some people doing it and getting away with it. However if you are caught you can be fined and unable to order supplies. Hope that helps!