I took my Non-Machinable to the local PO and showed it to the guy. (This is not the same as the other PO that sent the envelope back postage due) He looked at it, bent it, weighed it and hand canceled.

Three more going out today, one was close so I just threw a $5 stamp on it and gave up.

That's what I got back from USPS when I asked why my Non-Machinable was returned postage due. It's a nice hand flow chart for deciding the postage and their system.

I sent this and got back, no answer, except, case closed.