This blog http://dottyandalice.com/ linked to your username is one we provide support for on this forum, which is only for support for blogs being hosted by WordPress.com. There is no upgrade you can purchase from wordpress.com that will provide the ability to upload plugins. All blogs hosted anywhere else are supported on this forum > http://wordpress.org
There is no FTP access to free hosted WordPress.COM blogs like the one linked to your username and we cannot install individual plugins or third party themes into them. Those found on the internet are for WordPress.ORG installs which run on different software.
I’m afraid e-commerce isn’t currently possible here at WordPress.com as our free hosted blogs cannot be equipped for ecommerce transactions at this time. If you require e-commerce/shopping cart functionality, there are no restrictions on e-commerce if you setup a self-hosted WordPress.org blog instead of a free hosted WordPress.com blog. See here to learn the differences: http://support.wordpress.com/com-vs-org/