I have a 13 oz book that is 14 oz after wrapping, but PBS suggested that I put $4.85 worth of postage for media mail!  Now, maybe I'm not reading http://pe.usps.gov/text/dmm300/ratesandfees.htm right, but it seems to me that it would have to be over 9 pounds to cost this much and it should really cost $2.13.  PBS had the weight as 13.6 oz although it was 13, but even if it were over a pound it should only be $2.47.  Any insight into why this happened?

Katherine, here is a link to the Help Center on whether or not the PBS postage is correct..and what to do if it isn't.  Hope this helps:  http://www.paperbackswap.com/help/help_item.php?id=197

My guess is that the information was inputted wrong and that is the reason for the error.

Yes, sounds like the book weight is wrong.  I once had postage come up as over $90 dollars.  Hee hee hee!

You can correct the weight that shows on the printing screen, and it will adjust the postage amount.

One thing to know about the postage is that it is the suggested postage (or as in your case, is just blatantly wong), and you should always double-check the weight of your book before sending it out.  In fact, yesterday, I was instructed to put $1.85 on a book and ship it first class when it was actually heavier than the first class rate they quoted, bumping it up to $2.13 and media mail. 

That happened to me one time, so i went back to look at it before buying postage.  It had the book weight (standard MMPB) at like 8 pounds and like 6.5 ounces or something.  I didnt see the 8 pounds before, because it was a small book why would I look?  Anyway, just make sure pounds is 0

The guy at my PO always looks to see what PBS said the postage was.  He gets a kick out of it being wrong.