Device Tracker Plus is a monthly subscription service and so you will make payments once per month on the same day that you subscribed. So, for example, if you created your account on June 10th you will make your next payment on July 10th. Payments are automated and will be applied to the card you provided when you subscribed for the service.
We try to make charges as clear as possible for customers which is why we clearly state that it is a per-month fee when you subscribe. We also send you an automated receipt when you register, reminding you it is a monthly fee.