Thank you for reaching out to the community!
I'm sorry for any confusion with how Constant Contact billing works. The terms of a prepaid service with Constant Contact are clearly explained at the time of checkout, that you will be automatically charged at the end of your paid service if no action is taken. You can find our terms of service right here if you would like more information about this.
To make sure that you get the notification that your account balance is running low next time be sure to update your billing email address. This information can be found by visiting "My Account" and click on "edit billing address" near the bottom left.
I hope this helps! Please let us know if there is anything else we can do for you.