Mileage allowance for self-employed persons in secondary employment

Yes, invoice it as a sales item. I think the difference with working as an employee is that the mileage allowance is tax-free. You will probably also receive less from your employer than what you invoice as a self-employed person? (Google mileage allowance ;-):

Do you travel to work using your own transport? Then your employer can reimburse you up to €0.21 per kilometre without you having to pay tax on it. For this maximum mileage allowance, it does not matter what means of transport you use.

For the period from 1 April 2023 up to and including 30 June 2023, the maximum tax-free mileage allowance amounts to €0.4246 per kilometre (amount subject to confirmation by the FPS Finance).