You can definitely use eCart to edit the cart functionality in Power Store. If you have eCart installed you will see that the PowerStore cart object is available in the eCart object panel. To make a new cart display you should start by making a backup of your existing cart.php page. Then you will need to remove the existing cart display from this page. Once it has been completely removed you can add in a new cart display from the eCart insert bar and choose the options you would like for the display.