Please go to the Store -> My Upgrades section of your dashboard, where you'll find the Custom Design upgrade in the list of your upgrades, along with a "Cancel" button.

Unfortunately, your domain has passed the 48-hour refund policy on registered domains. Please note that this is different from other upgrades and is stated during the purchase process.

Canceling a domain after 2 days of purchase means you completely give up ownership of the domain. There is no guarantee the domain will become available to re-register after cancellation.

You are welcome to keep your domain with us and update the nameservers to point to another hosting provider by following this guide:
http://en.support.wordpress.com/domains/change-name-servers/

After 60 days from the date of purchase or registrant contact information change, you may also transfer your domain to another registrar.
http://en.support.wordpress.com/domains/transfer-domain-registration/

Alternatively, I could just disable auto-renew on the domain for you.

If you would like to completely cancel your ownership of the wedontdoradio.com domain name, please reply to confirm.