SimplePay charges for the exact number of active employees a month.
There are no extra fees other than what you see here.

Weekly / fortnightly employees are charged the same as monthly, i.e. not per payslip, but just for the month.

Inactive employees are not charged for, i.e. if someone's service is ended in March, you will not be billed for that employee from April onwards, even though we keep the data of this employee around indefinitely.