For those who care, here are the official post office physical standards (pictures and all) for mail:
101 Physical Standards
http://pe.usps.com/text/dmm300/101.htm#wp1039555+
(see 2.3 for the flexibility issue with first class flats (large envelopes))

It is my feeling, as expressed by others too in the thread, that:
¶ the post office clerks are loathe to classify much as "flats" (a/k/a "large envelopes" -- the 88-cents for one ounce class), due to the flexibility and/or thickness issues [bubble wrap].
¶ but if you throw it in a collection box, it will almost always go through without a hassle.
¶ I like to generally sandwich the chips between thick cardboard for safety reasons, so I almost always use the "parcel" rate ($1.22 for first ounce).
¶ for inexpensive strong chips (like modern chips), I wrap them neatly in paper, and then tape that paper package to the inside of an envelope, so it is no more than 1/4" thick -- and can use the "first class LETTER" rate of $0.44, plus $0.20 surcharge, for the first ounce.

Robert