MarketPress grid item order

Is there a way to order the items by price in reverse order?
You can see the site here: http://boaztrust.org.uk/store/

Forgot to say, I've tried both:
[mp_product_grid order_by="price" order="ASC"]
and
[mp_product_grid order_by="price" order="DESC"]
And neither changes the order...

Any ideas?