“do they think POTUS opens all the packages sent to the WH?”
Maybe they think that a Newman-type carrier be-bops through Security and drops mail at the door to the first family’s living quarters.
(Or ... worst case: It was sent by FBI / CIA / SS.)
I believe the point of mailing the poison was NOT that they expected the President to open the package, it was an act of TERROR ! A let’s “rise up” and kill the President clarion call.
The mainstream news stations would certainly be DELIGHTED and hyperventilate about the assassination attempt, and would likely subliminally encourage future efforts and try to expose weaknesses in White House security - “NEXT time, guys, use an RPG !!!” ( NOT to be confused with RBG, another kind of destructive unguided missle .. . )