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You can disable auto-renew of any upgrade > Store > My Upgrades
click the "disable auto-renew" link.
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
See also http://en.support.wordpress.com/bundles/#canceling-a-bundle
Note: The refund can take 1-2 weeks to post back to your account.