How to cancel payment on iTunes?

I downloaded an app (game) using iTunes, and I paid $2.99. After I got it, I noticed it is only for iPad, but I don't have iPad, I only have an iPhone!

I did not see (I forgot to check) when downloading the app that it is compatible with iPad only! I only saw this after I downloaded it... :(

The misleading part was that the icon I clicked in my browser (that forwarded to iTunes) was a picture of an iPhone!!

So... my question is: how do I get my money back?

I know it's not a lot of money, but... what if this happens with a more expensive app?

I've seen a lot of comments for other apps, where people say "I want my money back"; this seems to be the only place where you can buy something where you can't get your money back if it's not what you thought it is.