A free app charged me $2.99 for no reason. Help!

Recently I downloaded an app called skywalk on my iPhone. It has two versions, one free and one requiring $2.99. I was pretty sure that I got the free version, but after two days, I received an email that informed me that $2.99 was charged. I thought that I could accidentally download the charged version, but when I went to app store and checked, the charged version still required $2.99, which meant that I did not buy that one. Did anyone encounter the same problem? How can I get my money back?