Mich. Comp. Laws § 141.605

Sec. 5. "Doing business" means the conduct of any activity with the object of gain or benefit, except that it does not include: (a) The solicitation of orders by a person or his representative in the city for sales of tangible personal property, which orders are sent outside the city for approval or rejection and, if approved, are filled by shipment or delivery from a point outside the city. (b) The solicitation of orders by a person or his representative in the city in the name of or for the benefit of a prospective customer of a person, if orders by the customer to such person to enable the customer to fill orders resulting from the solicitation are orders described in paragraph (a). (c) The mere storage of personal property in the city in a warehouse neither owned nor leased by the taxpayer.