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seller to the purchaser at a point outside Illinois so that the sale is exempt under the Federal Constitution as a sale in interstate or foreign commerce. Nothing in this Section shall be construed to authorize the Authority to impose a tax upon the privilege of engaging in any business which under the Constitution of the United States may not be made the subject of taxation by this State. (c) If a tax has been imposed under subsection (b), a service occupation tax shall also be imposed at the same rate upon all persons engaged, in the metropolitan area, in the business of making sales of service, who, as an incident to making those sales of service, transfer tangible personal property within the metropolitan area as an incident to a sale of service. The tax imposed under this subsection and all civil penalties that may be assessed as an incident thereof shall be collected and enforced by the Department of Revenue. Beginning December 1, 2019 and through December 31, 2020, this tax is not imposed on sales of aviation fuel unless the tax revenue is expended for airport-related purposes. If the Authority does not have an airport-related purpose to which it dedicates aviation fuel tax revenue, then aviation fuel is excluded from the tax. The Authority must comply with the certification requirements for airport-related purposes under Section 2-22 of the Retailers' Occupation Tax Act. Beginning January 1, 2021, this tax is not imposed on sales of aviation fuel for so long as the revenue use requirements of 49 U.S.C. SB3019 Enrolled LRB104 20255 HLH 33706 b Public Act 104-0468