I would like to add commission to the Sales Invoice

Firstly create a new rate code as a commission rate code.

Then on the Assignment the On Costing tab holds the commission information for the timesheet.

Selecting the new commission rate code as above for the rate and the for the On cost & commission type select Client Pays.

The timesheet will show the commission as another bill costing rate only.

And the invoice will show the commission as a separate line.