Is there any reason paperbacks can not be shipped at the "flats" rate?  The only downside I see are:

* delivery confirmation can not be used with "flats",

* hardcovers can not be shipped at the "flats" rate because they are not flexible.,

* paperback must be less than 3/4" thick

http://pe.usps.gov/text/dmm300/101.htm#wp1002686

I doubt that most paperbacks would pass the flexibility test of section 2.3 in the link that you posted.   Even my thin paperbacks would be ruined if I tried to bend them an inch with the center rested on a flat surface.

I have been testing my paperbacks and some pass the flexibitlity test and some don't.  It is mostly the small thick books which won't pass.  (I damaged one of my small paperbacks when I performed this test.)

since it still would be a first class flat vs. a first class parcel. Why take the chance of:

a) damaging a book

b) having the person who requested the book pay the Postage Due when it gets marked as such.

c) break my flat sorter machine

d) dimensions of small paperbacks and what you put them in.

Kevin