Jigoshop are doing it wrong…
Instead of adding a page to the menu, you can add a link, directly to your ‘product’ archive page.
You could, just add a custom link that points to the archive page, though – this has its drawbacks… notably if the url ever changes you’ll have to manually update it and it will also not stay ‘current’ when you view individual post of that post type.
To get round this I’ve created this very small plug-in. It adds an extra metabox to your Appearance > Menu page, which lists your (archived) post types. You can use this to add the product archive link to your menu.