Marketpress Cart issue when www is used

I'm having a slight issue with the shopping cart/checkout page. (store/shopping-cart)

When I access the page without the www and I have products in the cart, everything behaves correctly
http://myfootballmentor.com/store/checkout/

When I acces this page with a www, and I have products in the cart, I get a message saying that the cart is empty.
http://www.myfootballmentor.com/store/checkout/

I've changed the slugs to see if that was causing an issue, but no dice.

Can you guys shed some light on this one?