The system will automatically bill you for your account each month, billing for both your monthly subscription and your SMS usage. Your subscription fee is determine by the number of diaries that are active within your account at the time of billing. You will also be billled for your SMS usage in the month just gone.
To see a breakdown of your invoice and determine how much was charged for both your subscription and SMS usage go to Setup > Subscription > Invocies and Receipts. Under the Summary column your total invoice amount will be broken down into subscription and SMS fees.