USPS is a no go because they often subcontract cross country transport to other carriers. They have no idea if the package will end up in an aircraft. That's why they ask you about "hazardous materials" at the post office. They don't have the infrastructure to guarantee that the package will only move by ground.
That being said, I have bought primers and ammunition on GB that was delivered by USPS, so some folks don't seem to care.
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