once you have ecart installed, you will need to make sure your dreamweaver site is defined correctly to make modifications to power store.
The Powerstore files need to be at the Dreamweaver site root.
The open the cart.php page and go to the server behaviors panel. In the list of server behaviors, double click the eCart object server behavior to edit the eCart object.
you will want to delete the existing shipping rules before creating new ones.