Replying late ---
The change in rate "structure" was at least partially brought on by new machinery, and doing away with old machinery. The shape test is because rectangular mail can be "faced" by machine - facing is orienting the envelope with right side up, address in front. The flex test is because rigid mail has to be handled manually for the most part to avoid jamming up the modern machinery.
In our specific case:
We mail floppy disks in a #9 envelope. There are two chipboard pieces to protect the disk. Will probably fail the flex test, resulting in the item being classified as a parcel.
We mail CD's in a 6" square chipboard mailer. Not only will the square shape cause a class/rate change, a CD is not going to pass the flex test, so this will also be a "parcel".
Each item we mail will cost us 1.30 for US postage instead of 63 cents.
We thought about going to a larger but rectangular envelope, but until the flex test is actually fleshed out in real use, it is not likely a CD will sneak through as a letter. It has been a bit since I read the specs, but my memory says the piece has to flex something like an inch. There is supposed to be a setup at each post office where a mailer can flex test their own mail if they choose to.
I am STILL looking at what will happen to Netflix and Blockbuster as far as their postage costs. If they can get through as a letter instead of a parcel, then maybe we can too.
Added:
I just revisited the issue. Our items all fall into the "letter" catagory as far as overall dimensions. We "fail" the machinable tests, so we are subject to the .17 nonmachinable surcharge.
For the curious:
Our floppy package, #9 envelope. The piece has to pass the flex test (have no idea if it would). 40 pound tension through a 11" radius curve. More likely we would fail the uniform thickness test, but maybe not.
The CD package meets the basic letter size, but since it is square, fails the aspect ratio test. Would also likely fail the flex test.
If I am following correct,y our 2oz pieces are now 75 cents (.58 plus the .17 nonmachinable fee) and will not be bumped into the parcel catagory.