HT5582: iTunes Store: How iTunes Store purchases are billedLearn about iTunes Store: How iTunes Store purchases are billed
Is the app working as described in the store ? You can try the 'report a problem' link from your purchase history : log into your account on your computer's iTunes via Store > View My Account (Store > View My Apple ID on iTunes 11) 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 app, 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 try selecting 'inadvertently purchased' as the problem and fill in the details (iTunes support should reply within, I think, about 24 hours).