When finalising the purchase (if you are buying from Amazon itself) a page appears on your way to the checkout asking you choose which type of delivery you want.You have to click the "Free Super Saver Delivery" to get free postage,otherwise postage WILL be charged.However,if you make a mistake and don't click the right postage,go into "My Account" and delete the items,and then purchase them again remembering to click the right postage this time.
I hope this is clear?
Amazon Marketplace purchases always incur postage.