Sec. 2. (a) The state gross retail tax is measured by the gross retail income received by a retail merchant in a retail unitary or bundled transaction and is imposed at seven percent (7%) of that gross retail income.
(b) If the tax computed under subsection (a) carried to the third decimal place results in the numeral in the third decimal place being greater than four (4), the amount of the tax shall be rounded to the next additional cent.
(c) A seller may elect to round the tax under subsection (b) on a transaction on an item basis or an invoice basis. However, a seller may not round the tax under subsection (b) to circumvent the tax that would otherwise be imposed on a transaction using an invoice basis.