As implied by its name, a subscription is a continuing process and as such, the charge is recurring and you are automatically rebilled at the end of every subscription period until you decide to cancel your account.
In other words, if you signed up for a monthly subscription, a charge will automatically be placed on your credit card every month on your renewal date. If you want to know when your next renewal date is, go to Your Account information page (click on the "My Account" link in the top left corner of every page on the site) and you'll see a date listed next to the "Expired Date" heading. This means that your subscription is paid up to that day and your card will be charged again on that date.
If you wish to cancel your subscription, just click on the Cancel option and follow the on-screen instructions. If you have trouble using the Cancel feature, just contact us and we'll do it for you (make sure you include enough details to let us identify your account and verify that you are the card holder.
Please note that cancelling your subscription only prevents the next charge from kicking in: it does not suspend the service that you paid for in advance. Your subscription will still be active and fully functional until the Expired Date listed on Your Account information page. On that date, your account will cease to work and you will not be charged anymore. You don't have to wait until the last day.