You are subject to their discretion if you present the mailpiece to them. The best way to avoid them is automation, but that thwarts the purpose of nonmachinable items.

These are the costs of trying to do things the cheapest way.

As an experiment years ago, I successfully mailed letters to myself with inadequate postage (1 cent), foreign stamps, "This is not valid postage" sticker from stamp books, and Easter Seals. I always had a typed address, no return address, and deposited them in a box to reduce the chance that a human would examine them. But I was not mailing anything of value - it was a blank sheet of paper and an experiment. You can get away with a lot, but is that the way to treat a trading partner or customer?