I would use a biological weapon, a virus with no cure.
The ventilation system will be the source of infection.
And of course I will disappear before anyone noticing anything.
Your fortune: Outlook good
I mean a fucking post office with a just a single (maybe even unarmed) security guard is also a government facility. So any gun is good as long as its not some muzzleloeading flintlock ez pz lemon squeezy