When you signup for the monthly subscription, the 99¢ signup fee gives you 330 image credits to start with. If you are a new customer, you will also receive an additional signup bonus (currently 500 credits).
At that point, your account has a "positive" balance, and will count down until you reach zero. Then the system will begin keeping track of how many image credits have been used for the next billing cycle.
Every month, your subscription will "renew", if you've used over 3,000 credits, then it will charge the payment method you provided at signup. If you're below 3,000 credits, or have a "positive" balance, no charges will be issued. The cost per credit will always be the same as when you signed up. If prices rise, your subscription remains at the same pricing level.
There are no minimum charges, credits never expire, and there are no limits to how many credits you can use in a month. You can, however, create "sub-keys" connected to your account, which can be useful for client sites, or for limiting the amount of credits that a site can consume in a month.