Amazon Product Advertising API Developer's Manual page 144

Product Advertising API Developer Guide
Shopping Cart Concepts
As you can see in the preceding figure, an ASIN identifies an item but not an instance of it that a customer
can purchase. Every ASIN can be sold by many merchants. Each merchant lists the condition and price
of the item. This combination of data represents an OfferListingId. Taken together, all OfferListingIds are
referred to as offers. If an ASIN does not have an OfferListingId, the item cannot be purchased.
For this reason, it is recommended that you add items to a shopping cart by their OfferListingId rather
than by their ASIN.
Remote Shopping Carts are Hosted by Amazon
Although a Product Advertising API application or web site runs on your servers, Product Advertising API
remote shopping carts are hosted by Amazon servers. You use Product Advertising API operations to
create and modify the cart and its contents. Keeping a local copy of a shopping cart is not recommended
because the price and availability of items change often and Amazon automatically updates items in
shopping carts. Also, Amazon automatically updates the status of lists, such as Wishlists, when a customer
buys an item on a list. Keeping a local copy of a shopping cart runs the risk of cart items getting out of
sync with their real price, availability, and status.
Because Product Advertising API shopping carts are hosted by Amazon instead of a local host, the
shopping carts are called "remote." The cart used by a customer shopping on the Amazon retail web site,
www.amazon.com in the US locale, is considered the "local" shopping cart. Amazon maintains for each
customer ID only one local shopping cart. Developers can maintain more than one remote shopping cart
for a customer but maintaining one cart per customer ID is recommended.
Cart Lifespan
Shopping carts have a lifespan. A remote shopping cart is created and, in time, it expires. In between,
the contents of the cart can be modified and purchased, as shown in the following diagram.
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