Cardboard (paper board) = 1 layer of paper.

Neither will prevent a clay composition chip to fracture/crack when USPS equipment decides to gobble up an envelope.

I'd go with cardboard before corrigated. It's much stronger.

It's all luck...

I wondered what to call it. I went back and forth with someone in the business and his words were "we don't say cardboard". So I asked what is cardboard, he said, "I don't know, but it's not corrugated"

I figured card stock, and that's one layer. Or as you added paperboard is a good one.

When it comes to the machines that mangle things, it is amazing to me sometimes. I mean people have mailed me chips in a business envelope. How did I get them?

By the way, my losing entry for the Survivor contest was mailed in a cardstock envelope, not corrugated. I have a bunch. When I mailed some chips to someone last week, they dropped through the slot at the post office. Barely. Corrugated wouldn't have passed the thickness test. Lady local for the four chips, didn't measure it and it went for what I assume was the reduced rate.

Once it doesn't bend or is over the 1/4" we pay the premium to have a machine mangle the cardboard, bubble pack or card stock. So I ask, why do we pay extra? Isn't that extra for mail that can't be run through the machines? Non-machinable mail, but it gets run through anyway. LOL Somehow I feel like we are paying for bureaucracy and regulations.