Why do I say if? Because all of the envelops shown on the news show packages without the common post office cancelled stamps. Also, the $3.00 of stamps would not be enough to mail out the metal pipe bombs shown in the photos. It would not be nearly enough.
Personally, I have never received any package through the US Mail without some sort of Post office markings on them.
Here are 3 of the photos that took me all of 15 minutes of internet search to find them (I am sure there are more):
These are all from screen shots
Even the left leaning New York Times expresses doubt about these packages. https://www.nytimes.com/2018/10/24/nyregion/clinton-obama-explosive-device.html
Whatever you do - Stay Safe!