MarketPress custom checkout page

Hi,

I know, to create a custom checkout page, I need to create a mp-cart.php template. Then insert mp_show_cart('checkout', $wp_query->query_vars['checkoutstep']); shortcode to include the checkout pages. This works fine, except functions, such as Update Cart, or Apply Coupon don't work. Is there something that the template file needs to include, like marketpress.php?