Most of the people on these forums, including myself, are fellow users - you're not talking to iTunes Support here.
Try the 'report a problem' link from your purchase history : log into your account on your computer's iTunes via the Store > View My Account (Store > View My Apple ID on iTunes 11) drop-down menu option and you should then see a Purchase History section with a 'see all' link to the right of it ; click on that and you should see a list of your purchases ; find that item, click the arrow to the left of it, click the 'Report a Problem' button :
and then the 'Report a Problem' link on its line :
and select 'inadvertently purchased' as the problem and fill in the details (iTunes support should reply within, I think, about 24 hours).
Or (the above sometimes doesn't work) you can try contacting iTunes support via this page and see if they will credit or reunf you : http://www.apple.com/support/itunes/contact/- click on Contact iTunes Store Support on the right-hand side of the page.