Well, private courier services (UPS and FedEx) can open any package for any reason whatsoever ... they don't even need a reason. And if they find anything suspicious, they can call the cops who can do a field test and send samples to a lab for testing, and the whole search is legal. The 4th Amendment does not apply to FedEx or UPS ... or an airline that opens a checked or carry-on bag, for that matter. The 4th Amendment only applies to actions by the federal government, but this was later extended to also apply to state and local government agents.

USPS is very different. The 4th Amendment applies for domestic mail (and international incoming mail after it clears Customs), because USPS employees are all government agents. So USPS cannot open First Class, Priority Mail, or Express Mail without a search warrant based on probable cause. And mail carriers are required to keep customer information confidential. They can be fired for blabbing about someone's mail. They are not supposed to question you about your mail ... and you don't have to answer them, and you certainly have no obligation to tell them the truth if asked. (Hazardous material shipments excepted.)

Incoming mail going through Customs is a different story. Any "man, burden, or beast" can be inspected by U.S. Customs when it enters the country ... no reason required ... and the 4th Amendment applies to only a limted extent ... like supposedly protecting people from strip searches without any reason, and preventing Customs from doing strip searches and body cavity searches based simply on ethnic background or meeting a "profile." Customs needs to have an articulable reason based on facts to do invasive searches on a person. After lawsuits from people claiming invasaion of privacy after body-cavity searches based only on the color of a traveler's skin, they now have a policy to try to prevent their agents from doing it. But whether they follow it ... who knows?

(Also, recent changes in federal law allow Customs to search *outgoing* mail.)

People are different from baggage and packages. Baggage and packages have no expectation of privacy when entering the country ... and persons have only limited privacy protections from Customs (according to both law and policy).

The 4th Amendment protects us from "unreasonable searches and seizures" ... by the government. A postal inspector with a package that meets the "drug package profile" can put it aside for a drug sniff ... and, if the dog alerts, then they can get a search warrant to open it ... that's legal.

But an airline ticket or baggage agent (or an employee of a courier service) who just has a hunch about a bag or package can just open it without calling the cops. Any evidence found can be used in a court of law.