Those "Free" boxes are only to be used for USPS business. From the USPS:

The terms of Agreement for the use of United States Postal Service shipping supplies is as follows: I understand that Express Mail service, Priority Mail service, Global Express Guaranteed, Express Mail International and Priority Mail International packaging is the property of the United States Postal Service and is provided solely for sending Express Mail, Priority Mail, Global Express Guaranteed, Express Mail International and Priority Mail International. Misuse may be a violation of federal law.