Does the partner have a share in the commission you receive from the auto parts store, knowing that customers give you money to bring the parts, and that the partner owns 20% of the workshop?

If the company's activity includes providing spare parts, or if they were purchased in the company's name, then both partners share in the commission. However, if the company's activity does not include this, and the purchase was made in a personal capacity, then the partner is not entitled to any part of the commission.