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Shopping Cart is a crucial part of your online store, just like a cart in your local supermarket, this is where your customers add products and buy them through checkout steps. Shopping Cart is not enabled by default but you may add it for free from the SiteGUI Appstore.
Once you have purchased the Shopping Cart app (free), you can click on the App Listing menu and choose Apps to look for it. Click on Activate if it is not activated or choose Configure to configure your Shopping Cart.