In addition to your monthly subscription fee, additional usage fees may apply when exceeding your monthly allowance for certain tools. You can see a full breakdown of the usage allowances on each tool here.
Raven bills for the maximum usage per month. So, for example, Pro accounts are allotted four sub-users total. If you were to add two additional sub-users at $19/month each, you would be billed for $38 total at the end of your billing period — even if you deactivated one of those users.