So, don't ask me why I know this, USPS typically needs to have some sort of reason to check your mail - the warrant argument is true, but...type of thing. Like if they suspect you of something - they gonna open it; or, if you're being investigated they will open it. But, it is actually a little safer than Fedex or UPS because they are private companies and can search something whenever the fuck they feel like it.
The key is to always mail packages from different locations, pay the few in cash and try not to do something suspicious lol.