I apologize for any confusion you may have about the charges on your account. Have you recently added a large ammount of contacts? Or switched the type of email plan you are in? Either of these two actions can cause your monthly charges to change.
While we cannot go into an account's billing details via the Community due to account security, this is something our Billing Support would be happy to look into with you. Here is a link to the Billing Department's hours and numbers.