The title of this post is the "unwarranted negative feedback" so I have myself opted to concentrate on that. It is unwarranted for couple of reasons:

1) The postage of $10.00 was clearly stated in the listing. If you are not happy with the charges simply don't bid, or ask prior to bidding if the charge is accurate and if there is a possibility of getting it cheaper. If the answer is no, simply don't bid.
2) According to the seller they never had an opportunity to explain the actual postal charges being lower then the amount charged. Also the negative feedback is no longer based on the actual item in question - book, but postal charges.
So is the buyer happy with the book, or was the seller selling the postage here?
Sellers have little, or no rights on eBay and it takes 2 to tango. Negative feedback is a tool that should be used very carefully and only when it is warranted and justified. Otherwise it speaks and reflects badly on both the seller and the buyer.