You're local PO is unfortunately correct from all of my experiences. There's a number of ways to ship a single in a normal letter envelope and pay around $0.70 (I forget the exact amount) with the non-machinable stickers they have (though there's other ways to go about doing it that may be a few cents cheaper?). Many PO workers though will feel the envelope or ask you what's in it and if you say a chip or coin many will say you need to pay first class mail. I believe USPS website lists the various things you can mail via envelope and goods like chips are not on it. So the name of the game when packing it, for me, is to get the envelope as consistent as a width as possible with one of the following ways:
1) put the chip in a dime baggie (small ziplock that fits a single chip) or some kind of other thing wrapping, take a piece of paper or thin cardboard like a cereal box cut to just fit the envelope, tape the chip in the baggie to that and fold it in the paper/cardboard (so fold in 3rds for paper or in half for cardboard) allowing the envelope to be as this as possible but making the width more consistent throughout
2) take a piece of normal corrugated cardboard (think Amazon boxes), cut the cardboard to the size of the envelope then trace and cut a circle the size of the chip in the middle of it, place the chip inside the hole then cover with pieces of paper on each side, tape the paper coverings to the cardboard so the chip will remain in the hole
3) toss a chip in an envelope and hope a) the PO doesn't care and b) when they try to put it through a machine or in general envelope transit, the chip doesn't pop through a corner of the envelope (I've been on the receiving end of this multiple times)