As part of new tariffs on Chinese imports, President Donald Trump eliminated an exemption for small packages, vastly increasing the amount of parcels US Customs and Border Protection needs to inspect.
The Congress shall have Power…To establish Post Offices and post Roads
The Post Office has the constitutional authority to designate mail routes. The Post Office is also empowered to construct or designate post offices with the implied authority to carry, deliver, and regulate the mail of the United States as a whole. The Postal Power also includes the power to designate certain materials as non-mailable, and to pass statutes criminalizing abuses of the postal system (such as mail fraud and armed robbery of post offices).
https://en.wikipedia.org/wiki/Postal_Clause