Is a state legally allowed to search and seize private post if something is against state law that isn’t against federal law? Or is that one reason the postal service is federal?
Items sent via USPS can be seized with a warrant. Items sent by other carriers do not need a warrant, just a suspicion.
From here on out, the Constitution says whatever Judicial Watch and the Federalist Society say that it says.
I'd expect packages from out-of-state pharmacies would be deemed "suspicious" and searched. And there's always the handy "exigent circumstances" (translation: We can do anything we want anyway).
I guess the good news is: It's unlikely there are dogs trained to sniff out this specific medication.
They can't do something like x-raying them? I'd imagine that would be relatively easy (e.g. bubble pack with X pills, of a certain size, a certain distance apart), to the point where it could probably be automated.
That's not why the postal service is federal though. Establishing a postal system is a power given to congress in the U.S. Constitution. "To establish Post Offices and post Roads;" Actually having a postal system run by the government was something that predates the U.S. constitution.