If you have a recent "pending" charge on your credit card statement, and the amount is for more than what you've accrued in advertising charges so far, don't worry! We haven't taken your money (yet)! This is a pre-authorization to charge your card later, up to that amount. We choose this amount based on your estimated spending over the next 24-hours (and some other factors - sometimes the amount can be less). However, this authorization is not yet a charge, and you won't necessarily be charged that amount unless your actual accrued advertising costs exceed it.
We will never charge you for more than what you've accrued in advertising costs (as calculated by how many impressions were served times the CPM bid rate for each impression; more info on that here). If you end up spending less than the pre-authorization amount, the final charge will only be for the actual accrued advertising costs, and the amount on your credit card statement will update accordingly. In this scenario, the card will be charged about a week after the initial authorization, and the initial authorization will remain "pending" on your credit card statement until then.