If you bought something by mistake, how do get your money back after?

Mac App Store: Frequently Asked Questions (FAQ) - http://support.apple.com/kb/HT4461

What is the Mac App Store refund policy?

If you experience a technical issue with an app, first contact the developer of the app for assistance. Their support contact information can be found on the app's page in the Mac App Store. If that does not resolve the issue, contact Mac App Store Customer Support.

To try and get a refund...

Contact the Mac App Store Customer Service:

http://www.apple.com/support/mac/app-store/contact.html?form=account

But it is stated that all Sales are Final...