MX is Fusion 6 under the hood. Since MX, there has been 7, 7.5, 8 and now 9...the current release.
Fusion 9 has some significant improvements over MX, including all of the improvements in the intervening releases.
Regarding your metatag issue...back in MX, you only had Site Settings to add site-wide tags to and the Between Head Tags tab on individual pages in the Layout and Master Border HTML.
Now, as I said in my previous post, in Fusion 9 you add metatags in Site View in the page properties.
Fusion 9 is available through the BeyondFusion store.
I've heard of people using Miva Merchant successfully with Fusion. It takes some configuring though.
More people probably use Mal's E-commerce. Often the free version is quite sufficient.
Fusion 9 also has an e-comm component included. It's suitable for small catalogs and has built-in interfaces for several shopping carts, including Mal's.