Hey, Chip, you just came in after the party ended! We're just sweeping up the confetti now. Giveaway #40 is up now, though. Just click on the cat picture above. Or the link in my signature line. Or here. It's like @masterswimmer says below. For all I know, you're right, and the USPS ignores it, but I pay extra postage for the "rigid" letter rate, so hopefully "nonmachinable" is presumed. I dunno. I do notice that old-school auctioneer Frank S. Robinson puts this on his coin shipments. He also uses an "FSR Metalworks" return address, presumably as a smokescreen to imply - but not declare - that small gears and washers and such are being shipped. Clever. Camouflage, but not a lie. I use card stock inside the envelopes so they are more rigid but not as thick or inflexible as a corrugated safety mailer. This means they will still flex a bit and hopefully still make it through if they go into sorting machinery. The small 2"x3" inner manila envelope holding the coin is taped so it is dead center in the middle of the packaging as it rides along, well clear of the bottom or outer edges of the mailing envelope. With even a 1-ounce item costing nearly four bucks to ship as a package nowadays, I will use the "letter" rate any time I can get away with it, but do take some basic precautions. Of course for more expensive stuff that needs tracking or insurance, I go with a bubble mailer at the package rate.