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.
http://en.support.wordpress.com/ftp-access/
http://en.support.wordpress.com/plugins/
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/
See http://en.support.wordpress.com/bundles/#canceling-a-bundle
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
http://en.support.wordpress.com/upgrades/#refunds
Note: It takes from 1 - 2 weeks for the refund to be received.