How do I invoice a client for the commission I have earned for selling their products?

it depends on what you want the client to see

Create an other charge item for the commission, select your income account, and enter the rate as a percent, ie 23% (include the % sign), set to non taxable

If they do not need to see the gross amount, on the invoice, select the commission item an manually calculate and enter the amount

If they do need to see the gross amount, then
Create a service item called gross sales, and select an asset account you create called clearing

on the invoice:
line one: select the gross sales item, qty = 1, and enter the amount of gross sales, non taxable
line two: select the commission item, qty = 1, non taxable
line three: select the gross sales item and enter a qty of negative one (-1), and the same gross sales amount, non taxable

The invoice will net to the commission due