This. By bidding in an auction or hitting Buy It Now you are agreeing to pay the price of an item and its postage, both are in writing. Otherwise you will end up eBay being full of dodgy sellers who put the postage into the item itself plus double GST plus a good percentage of profit on the side, add "free" postage to the listing and then send the item in the cheapest method of postage possible. Otherwise eBay will be full of $10 items with $100 price tags before we know it.