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. If your require plugins you must hire a web host and set up your own WordPress.org install. http://support.wordpress.com/com-vs-org/
WordPress.com provides a 30-day refund on all upgrades except Domain Registrations, Domain Renewals, and Guided Transfers. Provided you are logged in under the same username account that purchased the upgrades, you can request a refund yourself directly from the Store section of your Dashboard
Note: It takes from 1 - 2 weeks for the refund to be received.