If the vendor cannot be arsed to calculate a realistic P&P charge, and cannot explain the difference satisfactorily when asked, then it's tough if they get low P&P feedback ratings.
(I mentioned above, I accept the P&P is not just the postage fee, but also the packaging and the cost to the vendor in preparation and travelling to the post office)
The PIV example is not a very realistic example though. It's against eBay policy for the vendor to hide the true cost of an item in the P&P and avoid paying eBay fees. If I was to buy it, wait for it to arrive and then complain to Ebay and PayPal chances are I'd get a partial refund (effectively getting the item for $15) and the vendor would loose their P&P fee and their item.