Accidental purchase

Purchases are obviously considered final as you know, but you could try the 'report a problem' page to contact iTunes Support and see if they will refund you 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’)

This is a user to user forum: you are not talking to Apple here and we don't have access to your account.

First, cancel the subscription: please see View, change, or cancel your subscriptions - Apple Support

Strictly speaking all purchases are final (after all, you can't very well return them): please see this page for information on requesting a refund: Request a refund for an App Store or iTunes Store purchase - Apple Support

However Apple have been known to be sympathetic in cases of actual error, so it's possible that they may allow you a refund as a one-off concession: but please be aware that they are under no obligation to do so: go to iTunes Store - Contact Us - Official Apple Support

Alternatively go to the page suggested in the Help page linked to above: Report a Problem