When I signed up for an SEOmoz account I received several notices during the first month that explained the charges. Here is an excerpt from one such email:
"Also, we wanted to let you know that you have one week left in your free trial, and when it expires, you will be charged $99/month. We hope you've enjoyed your membership, but if you wish to cancel, you can do so on your account settings page."
I'm guessing your notices are in a spam filter.
Best,
Christopher