There is no FTP access to WordPress.com free hosted blogs. We bloggers cannot install any plugins. Those are for WordPress.ORG installs which run on different software.
There is no upgrade that you can purchase from WordPress.com that provides FTP access , the ability to install plugins, third party themes and software into free hosted WordPress.com blogs.
E-commerce transactions via shopping carts and the like cannot be conducted on free blogs from and being free hosted by WordPress.com.
On free hosted WordPress.com blog we can use a contact form and/or a PayPal donation button for sale of only products or services created or provided by ourselves.
There is no upgrade you can purchase from WordPress.com that changes any of that. If you do require an ecommerce site, shopping cart, advertising, affiliate links, paid or sponsored content you can hire a web host and get a free software install from http://wordpress.org
Refer to > http://support.wordpress.com/com-vs-org/