35 ILCS 110/3d

(1) Except as provided in paragraph (2) of this Section, the selling price of each item of tangible personal property transferred incident to a sale of service may be stated as a distinct item by the serviceman to the service customer and the tax imposed by this Act shall when collected be stated as a distinct item separate and apart from the selling price of the tangible personal property. If the selling price of each item of tangible personal property transferred incidental to a sale of service is not stated as a separate item on the serviceman's billing to the service customer, then the tax imposed by this Act shall be based on 50% of the serviceman's entire billing to the service customer.

(2) When a serviceman contracts to design, develop and produce special order machinery or equipment, the tax imposed by this Act shall be based on the serviceman's cost price of the tangible personal property transferred incident to the completion of the contract.