Final answer:
Confidential-level classified information may be transported via USPS mail.
Step-by-step explanation:
The level of classified information that may be transported via USPS mail is Confidential. According to the Supreme Court, the press may print classified material if it does not compromise troops or covert operatives. However, it is important to note that the transportation of classified information through USPS mail must follow specific protocols and guidelines to maintain its security. However, Top Secret and Secret information generally have more stringent handling and transportation requirements, often involving specialized courier services and secure transportation methods. Unclassified information, by definition, doesn't have any classification restrictions.