to use eCart to modify power store, you first need to make sure that the PowerStore files are at the root of your Dreamweaver Site.
Once you have the PowerStore Files at the root of the Dreamweaver site, open the cart.php page, then go to Window -> Server Behaviors.
In the list of server Behaviors, double click the Cart Object to edit it.
Go to the shipping tab. You can remove the existing shipping rules if you are not going to use them, then add a new shipping rule.
If it is going to be a true flat rate shipping rule, set the trigger to number of unique items greater than 0, and add your flat rate cost.