PowerStore 3 is a complete eCart site that is full functioning. It has all of the add to cart, cart display, and checkouts all ready incorporated. If you would like to add to the charge rules with your own advanced charge rules all you need to do is add them with the eCart object.
Before you do this with your PowerStore though you should make sure that you are working with the updated files so that you don't have any issues, here is a link to the post with the updated files:
showthread.php?t=12820