The way I understand it is as follows:
You could have secure certificats and shopping carts installed on every one of your domains, but you would have to pay for them. Currently Miva licenses are $595/domain. The strictly business plan includes a free Miva license that you could use for one domain.
If you have several sites that are going to need shopping carts, maybe you could create a cart for shopping and payment on just the one domain and seamlessly integrate it into your other sites.