Like many other merchants, the iTunes Store, iBooks Store, App Store, and Mac App Store place an authorization hold on your payment card account for the approximate amount of any purchase you make. If you update your billing information, you might see a nominal charge on your payment card account to verify your account information, for example, $1 in the United States or something similar for the currency in your region.
You might see authorization requests on your online statement. These requests aren't actual charges. They are tests to confirm that your payment card account is active and has the funds available to complete your transactions. Authorization holds are removed by your financial institution shortly after your purchase clears. The time it takes to remove authorization requests varies by financial institution.
If you have questions about the way your financial institution handles authorization requests, contact their customer service department.