Your invoice charges you for the upcoming 30 days of use for your stations. If you change your station limit, the next invoice will include the fees for the remaining time in this billing period for the additional stations. The additional fee is due to prorated charges.
Your invoice will normally be the number of stations you currently have multiplied by the set rate for the month moving forward. For example, 5 stations at $30/month would be $150 for your subscription fee.
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. So if you had 3 stations and added 2 additional stations during the month for a total of 5 stations, you will pay the $150 to cover all 5 stations for the following month and there will also be a prorated additional charge for station 4 and 5 from when they were added last month.