Some plugins which are listed in https://opensolution.org/download/quick.cart-and-quick-cart-ext-plugins.html, I do not see it in the site admin Tools -> Plugins. The plugin which I am reffering to is: googleSiteMap.
What version of Quick.Cart.Ext? In the latest v6.7-2021 this plugin is installed by default. Please check this: YourDomain/sitemap.xml or try this http://demo.opensolution.org/Quick.Cart.Ext/sitemap.xml