Can a post office generally open random packages under the guise of intercepting drugs?
I would very much hope not.
I would very much hope not.
https://www.uspsoig.gov/document/packages-suspected-containi...
FISA (post 9/11 "national security" authorizing legislation) adds some exceptions
> Third, provisions in the Foreign Intelligence Surveillance Act of 1978 (“FISA”), as amended, 50 U.S.C. § 1801 et seq., specifically authorize the Attorney General to conduct physical searches of mall without prior judicial authorization in certain circumstances. Section 304(e) of FISA, 50 U.S.C. § 1824(e), provides that the Attorney General, under certain circumstances, may approve the execution of an emergency physical search of property, including property that “is in transit to or from an agent of a foreign power or a foreign power,” id. § 1824(a)(3)(B), so long as the Attorney General subsequently obtains an order from the Foreign Intelligence Surveillance Court authorizing the search...
https://www.rstreet.org/2014/11/19/yes-the-government-can-op...
Likewise, I've received one package that was opened and inspected. It came in a clear plastic bag along with a very short note stating that it had been inspected in accordance to <whatever laws they were>.
Senders probably don't usually stamp that on illegal shipments, and the post surely doesn't drop in a polite notification if they discover something illegal in there.
Unless someone forged the notice (unlikely, as you point out) some government opened it (it could also have been the government of the sender or any transit country). All those notices are the government voluntarily letting you know because someone in the government thought it would be better; if the FBI wanted to open a package because they vaguely suspected terrorism they could do it without needing to stamp the package or give any notice if they wanted.
[0] https://www.uspis.gov/wp-content/uploads/2019/05/USPIS-FAQs....