How do I cancel a pending purchase?

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 it as a one-off (if it still hasn't been paid you might not be able to use this option): http://reportaproblem.apple.com

Or you can try contacting iTunes Support via : http://www.apple.com/support/itunes/ww