I searched the questions on the site and found a way to create a custom shopping cart, which I did here:
echo mp_items_count_in_cart().' Items, '.marketpress_get_cart_subtotal();
It just shows the amount of items in the cart, and the total.
It would be REALLY cool if, when I added items to my cart (clicking add to cart on products page), the the div was refresh via ajax as well.