No matter what Amazon's policies are, if this vendor did not state up front that you were to pay postage on the free item (and gave you an invoice with a total amount due that didn't include an additional shipping fee), then it was that vendor's error and he/she should have simply written it off and not tried to charge you after you had already paid. If it had been me and that vendor wouldn't budge on it, then I would have cancelled the order and would never do business with them again. I'd even go one step further and leave feedback that alerts other buyers as to their shady business practices.