Adding a new menu when user has selected a product

Hi there,

I'd like to add a new menu item (a button called 'Checkout' ) that only appears in the menu bar when the buyer has selected an item to purchase in the marketpress store.

I know that there's already a checkout button that appears in the sidebar widget but I'm getting feedback that some people miss it so I'd like to put it in the menu bar.

Help would be appreciated!

My best,
Rob