Why is my App Store and iTunes Store over charging me!?

Why is it when I try and buy an app to download the App Store is over charging me? Because I’m trying to buy buy an app that is $2.99, but my credit card account is notifying me that I don’t have enough to buy it. So when I went to check to see why, and how much it’s trying to charge me it’s saying it was trying to charge me $18.20. This has happened before too but it was for my work app but that app was supposed to be free. So I’m just confused with why it’s over charging me for something that is only $2.99. Also I did try to see if it would do the same in the iTunes Store and it does the exact same thing. So if someone could explain why this is happening I would really appreciate it.