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?