I think I have another book like this..label created, but that's it. It stays as created and USPS awaiting item. Eventually I get the have you received the book emails..( while this doesn't happen often, I've had it happen a few times)

I just take my books to the post office because it's across the street from a place I volunteer at every week. When this happens and the book isn't mailed, does the sender lose the shipping cost or does it get refunded?

If it doesn't get refunded, I don't get why those books aren't actually mailed.

If they paid for the postage when the label was printed, they lose the postage fee. But you can create a PBS label without paying postage, if you are then using postage you got at the Post Office, or an online vendor.  The latter is what I think most members do, as they don't want to pay the $0.55 charge. I know I usually only pay that charge when I'm shipping several books to one member. As a Standard Member I do use the once-a-month postage option  that doesn't include the $0.55 fee.