It is (under targeted surveillance) and it isn’t (practical at scale). My wife works at a post office and they do occasionally comply with law enforcement’s lawful requests to monitor or search the contents of a person’s mail, but it’s quite rare. It’s manual work that takes time and effort per-package, risks detection, and typically requires a warrant. It’s just not cost effective compared to automated electronic methods unless you’ve really fucked up and they’re already onto you.