Just writing “NON-MACHINABLE” on a envelope with a regular stamp does no good. You have to pay the surcharge for that extra feature.

You can go down to the PO and buy Forever non-machinable stamps so you don’t have to go to the PO again and again.

I also agree that you should invest a couple of bucks (Amazon) and buy a non-machinable stamp for the outside of the envelope.

The one I purchased is the one that mirrors the one that the PO uses.