I'm very glad this thread was posted. I though I understood pretty much all of the intricacies of mailing through the Post Office... but I did not know about the metal clasp turning the package into a non-machineable one (and hence an added cost). We use those clasp envelopes all the time for our first-class packages. But we never bother to use the clasp for closure. We just fold the flap over and tape it down. So we are taping over the clasp. Nothing has ever been returned to us, and none of our customers have reported receiving the envelopes postage due.
I'll have to check www.usps.com to see if they say anything about the clasps.
You learn something every day!