I know how you feel when you pay the extra .21 for Non-Machinable/ Hand Cancel.
My experience with the USPO was this, I actually handed the envelope to the clerk at the window with the Non-Machine in red on the envelope and paid the .21. When I handed it to the clerk she immediately
"flexed" the envelope back and forth to see how flexible it really was I guess. I was only mailing out some slot cards but they were secure. There is something you can do. I e-mailed a letter to the Main Postal Office and informed them to "Contact" the post office in my city and have the supervisor explain to his clerks just what "NON MACHINEABLE'. means. Not sure if my complaint ever will reach my local USPO, but I hope so. On my next visit to the P O if this happens again I will ask to see the supervisor just what it means right in front of the clerk, loud enough for other clerks will have no trouble to hear.