it is recommended that you add items to a shopping cart by their OfferListingId rather than by their ASIN.
90天remote cart過期
CartCreate -> return cardID
HMAC (Hash-based Message Authentication Code) is a specific construction for calculating a message authentication code (MAC) involving a cryptographic hash function in combination with a secret key.
http://docs.aws.amazon.com/AWSECommerceService/latest/DG/CreatingaRemoteShoppingCart.html
createcart 只需要呼叫一次