How to have additional menu sub-items in Marketpress

I've just installed Marketpress and in my menu bar I have a 'Shop' menu item with 'Products', 'Shopping Cart' and 'Order Status' sub-items automatically generated by Marketpress.
I would like to have two further pages as sub-items ('Returns' and 'Postage'). When I created these and put the 'Shop' page as parent they appeared but on top of the Marketpress generated sub-items. It looks as though two sub-menus have opened, one over the other.
What's the way round this?