Marketpress shortcode styling

I want to avoid long post slugs on my Marketpress store and to use my existing store structure, so am trying to use shortcodes where possible.

I’ve created a custom mp_productlist.php file which is working well, but when I use a shortcode the style and layout default back to Marketpress standard rather than my custom styling, even when I specify mp_productlist as the template.

Can anyone tell me what I need to know to make shortcoded categories use my own styling?