Subscription invoices cover the station fees for the upcoming month but occasionally there may be an Outstanding Balance listed on your Billing page. If you change your station limit, the next invoice will include the fees for the remaining time in the billing period for the additional stations. The additional fee is due to prorated charges.
If you added stations during the month, you pay an additional amount to cover the newly added stations for the days used prior to the current invoice. For example, if you subscribed on October 1st and paid for 2 stations for the month of October but on the 5th you added 4 additional stations to the account there would be and Outstanding Balance for the prorated amount owed for the 4 new stations you added after you subscribed.