Walt, thanks, I guess you are correct. I never noticed that before -- regarding "large envelopes (flats)" -- that asterisk in the chart (*) that I show above, and the notation that only ONE minimum had to be exceeded for flats. They sure make it complicated.

Anyway:
(1) one or a few sturdy, inexpensive chips -- I send in a first class "letter" envelope (not a bubble mailer). It won't exceed 1/4" thickness. Cost: 41-cents first ounce, 17-cents per additional ounces, plus a 17-cent surcharge. 3.5 ounce maximum fir first class letter. See picture below.

(2) valuable chips I always sandwich between thick cardboard, put them in an envelope and pay the parcel rate. Cost: $1.13 first ounce, 17-cents per additional ounces, no surcharges, 13 ounce maximum.

(3) I never send anything large envelope/flat rate because I am afraid that it will be kicked up to parcel rate due the rigidity, evenness issues. (But see what Archie advises.)

Robert