If it's pending iTunes hasn't got the money yet, so there isn't anything for them to refund.
Purchases are considered final as you know, but you could try the 'report a problem' page to contact iTunes Support and see if they will cancel its as a one-off : http://reportaproblem.apple.com
Or you can try contacting iTunes Support via : http://www.apple.com/support/itunes/ww (click on your country's flag, the country where you live, and which is therefore the country on the account ; click on Contact Support towards the top right ; click on Get Help under 'Talk To Us’)