26 U.S.C. § 4254: Computation of tax

the amount on which the tax with respect to such services shall be based shall be the sum of all charges for such services included in the bill; except that if the person who renders the bill groups individual items for purposes of rendering the bill and computing the tax, then (A) the amount on which the tax with respect to each such group shall be based shall be the sum of all items within that group, and (B) the tax on the remaining items not included in any such group shall be based on the charge for each item separately. If the tax imposed by section 4251 with respect to toll telephone service is paid by inserting coins in coin-operated telephones, tax shall be computed to the nearest multiple of 5 cents, except that, where the tax is midway between multiples of 5 cents, the next higher multiple shall apply. For purposes of this subchapter, in determining the amounts paid for communications services, there shall not be included the amount of any State or local tax imposed on the furnishing or sale of such services, if the amount of such tax is separately stated in the bill.