I had a book that came back to me today.  I used the PBS wrapper and used the site to pay for postage and delivery confirmation - just the same as I've done countless times before.  The receiver's post office said she needed to pay $3.29 on top of the $1.99 postage I already paid because you can't send items under 3/4" thick first class mail with DC.  This book was very light - less than 6 ounces.  He claimed it was a new postal law.  I've never heard of this before and neither had she.

Is there a new law to this effect or was this postal worker horribly misinformed?

From the Domestic Mail Manual 503 Extra Services:

Delivery Confirmation is available for First-Class Mail parcels, for all Priority Mail pieces, for Standard Mail prepared as Not Flat-Machinable pieces or as machinable or irregular parcels (electronic option only), and for Package Services parcels under 401.1.0. For the purposes of using Delivery Confirmation with a Package Services parcel, the parcel must meet these additional requirements:

a. The surface area of the address side of the parcel must be large enough to contain completely and legibly the delivery address, return address, postage, and any applicable markings, endorsements, and extra service labels.

b. Except as provided in 9.2.2c for machinable parcels, the parcel must be greater than 3/4 inch thick at its thickest point.

c. If the mailpiece is a machinable parcel under 401.1.0 and no greater than 3/4 inch thick, the contents must be prepared in a strong and rigid fiberboard or similar container or in a container that becomes rigid after the contents are enclosed and the container is secured. The parcel must be able to maintain its shape, integrity, and rigidity throughout processing and handling without collapsing into a letter-size or flat-size piece.

Was this a children's book, Roni? Most paperbacks meet the thickness standard.

Yes, it was a kids book.  Thanks for the info!

You can meet the thickness requirement by padding the book with cardboard or bubble wrap or tissue paper, etc.

Will do, thanks!

I've seen mailers put one small piece of styrofoam in the package to make it over 3/4" thick, It doesnt have to be 3/4" the entire package, just one part.