Adds Extended Properties

Add Extended Properties on the current cart. Store an arbitrary number of cart extended properties such as tracking strings, marketing sources, affiliates, sales personnel/data, and so on, on a per cart basis. Each cart may have none, one, or more than one entry in the extended properties collection, and all values in the extended properties collection are represented as strings. When you create an order from a cart, all extended properties are retained from the cart and copied to the order.