Product Advertising API
Developer Guide (API Version 2013-08-01)

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, 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.