Power Store 3 was built with eCart 4. eCart 5 can be used to modify eCart 4.
that being said, there is a lot of custom coding that when into Power Store. eCart was used as the Basis with custom coding added to accomplish some features.
It is posible that editing the eCart object using eCart 5 may break some of that custom code. Modifying the cart using eCart is not supported in Power Store.