Shopping carts and ecommerce are offered with all plans. We simply don't see the need to offer an Ecommerce plan specifically. Agora cart is offered in the control panel; as well as Oscommerce. We do recommend that you install a standalone version of the Oscommerce shopping cart. We have an overview of our Ecommerce Solutions here.
If you have access to a shopping cart and a payment processor, feel free to use it. We do not support it, however. There are plenty of free carts out there; and you are more than welcome to install them.