From the Model Mayhem FAQ:

Why was I charged the wrong amount?
Before we process your payment you are presented with a Subscription Detail page that tells you when you click submit what you will be charged. This page is an agreement that you will pay the amount shown. If you have selected the wrong amount, just click the back button to correct your order.

Here is an example of what that page looks like:

If you still proceed with the wrong subscription amount, simply contact a moderator and request help.