If you sell products with added tax/VAT, there are some options available in the “General” settings to decide whether to calculate commission including or excluding tax, and to even apply your own tax adjustments to the order total.
If “Show tax in orders and commission calculations” is checked, then it will calculate the commission including the VAT. For example at 10% commission, if the order is £100 + 20% tax (£120), with that option checked the commission they earn is £12. If it’s not checked then they get £10.
For the payout statements, there is an option in the settings to show the tax calculations on the statements.
You can also choose whether the commission payout amount should be shown as inclusive or exclusive of tax.