67.573.  Sales tax to be an additional tax to taxes in chapter 144 — computation of tax. — The order imposing the sales tax pursuant to the provisions of sections 67.571 to 67.577 shall impose upon all sellers within the area wherein the tax is to be paid an additional tax on all goods subject to tax included in chapter 144.  The amount reported and returned by the seller shall be computed on the basis of the tax imposed by the order as authorized by sections 67.571 to 67.577.  The seller shall report and return the amount so computed to the director of revenue.