Billings are all made upfront / in advance. These are sent every 15th of the month for every incoming period. Monthly billings are based on the active headcounts every 15th of the month.
For example, on August 15, 2017, the active headcount is 81. This will be the basis for the upcoming month's (September 2017) billing. So on August 15, the account will be billed based on that headcount.
We would like to emphasize to please make sure to terminate any separated employees on the system (on both Sprout HR and Sprout Payroll) on/or before the 14th of the month to avoid being billed for people who no longer work for the company.