This is something that I have been trying to get TPBST to correct, but they seem not to understand what I am telling them.  So I will tell you what the issue is, and then tell you how you can deal with it until it is resolved.  Short story:  the recommended postage is often inflated when the book weight is correct in the listing.  

If you weigh all your packages to get the mailing weight or let the post office do that for you, this message will have little significance for you.  If, as I have, you have made sure that the books on your bookshelf have the correct weights in the listings and are counting on PBS to give  you the correct amount of postage required, DON'T.  

When you have a book requested and go to mail it, PBS "intends" to add 0.6 ounces to allow for packaging when calculating the postage.  In reality, that is not what is happening.  The system adds 0.6 ounces to the weight in the listing to get the amount shown as "PaperBackSwap estimates your book weight at____ pounds_____ ounces."  If you accept that book weight, then the system adds 0.6 ounces to that weight to get the mailing weight for a total of 1.2 ounces.  This amount is added to each book in a multiple mailing, so that if you had three books, it would add 1.8 ounces to get the book weight and 1.8 ounces to that to get the mailing weight, for a total of 3.6 ounces.  

For many packages, this will not matter.  However, if you have a paperback that is being mailed First Class, a multiple-book package or a book that is near to the next pound (1.93 pounds, for instance) this can make a difference in the amount of postage that is shown on the wrapper.  For the example of 1.93 pounds, the system will convert the weight to 1 pound, 14.9 ounces, and add 0.6 ounces to get a book weight of 1 pound, 15.5 ounces.  Then, when it calculates the postage, it will add 0.6 ounces to get 2 pounds, 0.1 ounces and recommend postage for "up to 3 pounds" ($2.58), when, in reality, it should be recommending postage for "up to 2 pounds" ($2.93).  Granted, these amounts are not huge, but could add up if you mail a lot of books as many of us do.  

Until this is corrected, you may want to change the check box "Weight shown above is the ________weight" from "book" to "wrapped book."  This will have the same effect as intended; to add 0.6 ounces to the book weight to get the mailing weight.  I urge you to check the book weight shown on the wrapper page against the weight shown in the listing.  However, as always, the best policy is to weigh the package if you can.  

No guarantees that the weight shown in the listing is correct; I am on the data team, and we are correcting erroneous values and adding weights where we find them, but we cannot police 500,000 listings.  If you find a book weight that is incorrect or missing and you have the information, edit the listing.  

Note that this message is being created shortly before a postage cost change, so the dollar amounts may no longer be correct when you read it.  

Disclosure:  I admit that I do not know the exact amount that is added to the "book weight" to get the "wrapped book weight".  The 0.6 ounces is what PBS tells me is added.  However, some of my mailings suggest that it may be even more.  

I have sort of conducted an "informal survey" of books as I receive them and the vast majority of them -- and I am mainly talking about MMP (mass market paperbacks) here -- are carrying far more postage than necessary. So many of them are being sent via media mail, when they could have gone for 1st Class Mail, cheaper. Which means you would get your credits faster and the recipeint would get their books faster and it would not cost you as much! It is worthwhile having a scale -- so many people could buy one with the savings they would have if they could weigh their books before mailing and not go by the postage amounts estimated by PBS.

Wait a minute...  if I'm understanding you correctly Linda, then even when you weigh your wrapped book on your own scale, and enter that weight into the box and toggle it to "wrapped book weight", the system is still adding that 0.6 ounce extra?

Okay, that's not a huge problem, but could be enough to bump the weight to the next class or level, which would cost you more in postage even though you've weighed it correctly, and that's just not right.

Bernhard: No, when you put in the wrapped book weight, they are not supposed to be adding anything.  I don't know whether that is the case or not; I haven't checked.  You could check a few against the postage table to see.

What I was saying is that since they already added the 0.6 ounces to the "book weight" that you could use that for the "wrapped weight" since that is the weight that would be used for the wrapped weight if they weren't adding it twice. 

This is why I use DC only.  It's VERY RARE that the estimated postage printed on the wrapper matches what the PO determines is the correct amount. Sometimes it's more, sometimes it's less.  I live in a small town and the PO has no lines, so it's far more economical to me to take the packages in and buy my postage on the spot.  YMMV  I don't want to over pay and I don't want my package to arrive postage due!

I just noticed this on the last books I sent out. The weight was off by over an ounce so I went into the book data to edit it and realized that the number I saw on the mailing screen was .6 higher than the book data.  I thought it must be a short term glitch because the majority of my books weigh right at what the mailing screen says.

For the most part, I don't think it will have a massive effect on the system. To be bumped up an extra ounce will only make a difference in the FC rates and on the border of FC to MM and when a book just borders going over a pound. It would be good that they straightened it out though so the mailing screen showed the actual book weight again.

I have acess to a postage scale at work and have gone through and weighed all my books and then written that on a post it note. That way if I am at home and need to post a book I can do so without having to wait to weight the book. Some of them are already wrapped for posting so just need the address wrapper which hardly adds anything to the weight.

I do not rely on PBS telling me how much a books weighs as I have found one in the past that said a HB weight 5lbs when it only weighed 2lb.

And of course PBS is going to add a certain amount more onto the books weight when unwrapped to try and protect people from getting post due books.

I never rely on PBS weights. I weigh them at home and when configuring here, I do select "wrapped book" as the weight.  I also add in 1 oz. for packaging.