Free Apps from the App Store have on occasion have put a Mystery Charge on my Bank Account?

This Morning I downloaded the Pay Pal App from the App store which is free later when I checked my Bank account a Charge for $13.01 appeared on my Statement. This is not the First time it happened. But the amount has never been this high before usually its less then a couple of dollars never thought much about it 99% of my Charges are clearly marked so I don't know what's going on here.