USPS mail in a freight hauler? Maybe if it is important mail, a.k.a. Priority or Express ,etc. Probably not first-class or lower mail.
Well, if they had a couple Priority packages that absolutely have to be there overnight, they might as well throw in a few thousand pounds of first class while they are at it
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The deceased gentleman was their senior and most experienced pilot. I doubt that he just got careless looking at the map and flew into the ground. No emergency radio calls so he apparently wasn't trying to go around and land again (the wind has been from the east during all this rain, so he might have taken off "backwards" from the usual westward takeoff).
I'm suspecting some sort of catastrophic airframe or mechanical failure. Maybe a control surface failure or possibly a problem with the pitch on one of the props that threw him out of control. One of the witnesses said it looked like the plane was upside down just before impact, and another said it sounded like one or more props was/were reversing