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Select the content which is causing a problem and use the 'Report a problem' button in Your Purchase History on your computer.
Currently, if your purchase happens to be your most recent purchase you will not be redirected to the report form, in this case use the report a problem option from the invoice that was emailed to you.
You can check the purchase history on your account via the Store > View My Account (Store > View My Apple ID on iTunes 11) drop-down menu option on your computer's iTunes - though I'm not sure if that shows in-app purchases and/or auto-renewing subscriptions . If you don't recognise the charge and/or you can't find it then you can contact iTunes support via email via this page (I'm not aware of any phone support for iTunes) : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption