You get charged the second you click "buy".
iTunes Customer Service Contact - http://www.apple.com/support/itunes/contact.html > Get iTunes support via Express Lane > iTunes > iTunes Store - Apple states all sales are final, but you can always ask nicely.
post by stevejobsfan0123 - https://discussions.apple.com/message/18879347 - "In the iTunes receipt which will be (or maybe already has been) emailed to you, click on the Report a Problem link. After following the steps, one of the options is 'I inadvertently purchased this item.' Select that."