USPS cannot legally open your package even if they think they found weed in the mail.
The Fourth Amendment protects the rights of the people.
Mainly “to be secure against unreasonable searches and seizures.”
This means that if USPS finds weed in the mail, they can’t just open your package.
For USPS, first-class letters and packages are protected under the Fourth Amendment.
This means that postal inspectors cannot search or seize first-class packages.
But, let’s say that the postal inspector believes that you are mailing drugs in a package.
They can request a search warrant to open suspicious packages.
(But private companies like UPS, FedEx, and DHL can reserve the right to open packages.)
Related: Is Weed Legal In Florida?