I don't think that it is "common" ... but it does happen occassionally. While I've never lost a strike through the mail, I have lost a few chips in the manner you have described ... before I changed over to using bubble type padded mailers.... and I mail out literally hundreds of them a month with excellent results. I always put the chip or token in a cardboard 2x2, or inside a folded over bank envelope, or folded up in my invoice sheet. Never put a chip or a token in a regular letter envelope "bare" without some kind of wrapping to dull the sharp edges.

I have received a number of plain envelopes where a chip has been "squeezed" out of the end of the envelope, probably by cancellation and sorting machinery at the US Postal Service. I don't believe that most postal service employees would risk losing their well-paid jobs over stealing a relatively low value item.