It is common belief that all purchases on the App Store are definitive. Well, it's not the case. It's true indeed that you might get disoriented in iTunes but if you know how to proceed and as long as you have a valid reason, there is no obstacle to getting your money back.
1. Launch iTunes
2. Log on to your account (using the upper right box)
3. Proceed to Purchase history (iTunes link)
4. Report a problem
5. Fill out the form politely and concisely. You're going to have to provide a valid reason to be refunded
5b. In case you're unsuccessful or unable to access the form just use the online form, don't forget to give your purchase number and such
There are hundreds of stories around the web of people obtaining a refund because of a purchase mistake or because the app didn't function properly (like push notification wouldn't always go through). You also need to precise that a refund is what you would like to obtain.
After that you should receive an answer in the next 48 hours and you'll be refunded in 4 to 5 business days.
However, there is an issue with obtaining a refund. When Apple grants you a refund it keeps its 30%, so basically when you ask for a refund the developer is gonna be billed 30% of his app price.
Try to use this possibility with parsimony and be fair