I just received my Chip Guide chip from Charles. He used a CD/DVD mailer made of stiff cardboard. I think you can get these type mailers from Staples, Walmart, etc. for under 50c a piece in a pack of 25. He used a single 1st class stamp (55c).

So mailing cost is about $1 per single chip. Of course, no tracking.

Fine for "common" chips, though padded envelope offer more protection if sending nicer chips.