I want to cancel this order.

If you got an email receipt, use the 'Report a Problem' link in it. Otherwise, click here and ask the iTunes Store staff for a refund.

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As Niel posted, normally, all sales are considered final, but you can ask for a refund.

If you have received a confirmation email, click the Report a problem link at the bottom.

Purchases problem.   *

How long refunds from the App Store or other Apple services take.  *