I had a book returned for postage due the other day. I weighed it on 2 different electronic scales. It weighed 7.4 oz on each one.
I went into the Stamps.com interface to check to see what postage I had used ($ 2.07). I looked at first class/ 8 oz and saw it would be the amount I had attached.
I couldn’t see why the book was returned for $ 1.14 postage due.
I had been sending my paperback books out by First Class mail when they weighed less than 10 ounces. Larger than this, it was cheaper to send them ‘Media Mail’, so I sent them Media Mail.
I called the Stamps.com support center. I explained the situation & asked why US Post Office was suddenly not accepting the Stamps.com postage.
The helpful support tech. asked what kind of packaging it was.
I explained it was a impervious envelope/mailer around a paperback book.
He then asked me how thick the paperback book was.
I got out a ruler & measured it at 1 1/4 in. He stated that it qualified for “Thick Envelope” postage rate (for envelopes thicker than ¾ of an inch) which is higher than First Class.
He looked at the amount of postage due & was stumped as to why it was that high of an amount as it should have been less.
I made the comment that probably I should just mail all of my books that were thicker than 3/4 of an inch via Media Mail as it would be less hassle and possibly less expensive at times.
He replied that it would take longer to arrive than First Class. I came back with a statement of how the book swap was fine with slower shipping as long as it arrived with in 26 days. He then responded that
Media Mail would probably be a good idea.
I am surprised that after swapping hundreds of books, this was the first one that was returned for postage due. Granted, most of them weigh more than 10 oz so I shipped them media mail any ways…
I have been notified by PBS patrons that a book was delivered with postage due a few times. Each time, I sent them a book credit to make up for it. Since book credits cost $ 3.50, they were roughly $3 ahead. Most people were appreciative.
So I learned 2 things today:
1. Stamps.com support staff are courteous and helpful
2. The vast majority of paperback books will need to be shipped via Media Mail. 
