We gave up following the law when we let the Supreme Court disembowel our Constitution, once the supreme law of the land, many decades ago. Since then we simply run an "ad hoc" kind of government. The Constitution has become a "living, breathing document"; it changes with the changing of the judiciary.
In most (or maybe all?) western democracies there's a privacy/secrecy of correspondence which can only be breached under very specific circumstances and only by specially authorized people. Usually only one state agency can do that and others will have to go through them.
I read a bit about how it works in the US and for you it's customs that are allowed to do that at the border, nobody else (maybe your more secretive agencies NSA/FBI/CIA but I couldn't find anything on that).
In Germany only specific postal employees (or the recipient of the item themselves) can do that in the presence of customs officers (who picked the package), so there's 2 agencies involved (for mutual control) but only one allowed to open mail/packages.
I think rangerix is correct, FDA can tell customs to look out for something but FDA employees will never stand at the border and look for vape gear.
Customs are just following orders, if you want to change what they look at you need to go after the agency which gave that order.
If customs don't misbehave and act according to their orders there's no way to go after them.
It's like the police has to enforce the law and if they do so properly/correctly they're always in the right, you can only go after those making the laws.