A. Every corporation liable for income tax shall pay the same to the Department at the time fixed by law for filing the return. The full amount of the tax payable as shown on the face of the return shall be so paid. A corporation may file its return and pay its tax in full in the closing days of its taxable year provided it is able to prepare a complete return. If any payment is not made in full when due, there shall be added to the entire tax or to any unpaid balance of the tax, a penalty of six percent of the amount thereof, if the failure is for not more than one month, with an additional six percent for each additional month or fraction thereof during which such failure to pay continues, not exceeding thirty percent in the aggregate. The entire tax or any unpaid balance of the tax, together with such penalty, will immediately become collectible. Interest upon such tax or any unpaid balance of the tax, and on the accrued penalty, shall be added at a rate determined in accordance with § 58.1-15, from the date the tax or any unpaid balance of the tax was originally due until paid. In the case of an additional tax assessed by the Department, if the return was made in good faith and the understatement of the amount in the return was not due to any fault of the taxpayer, there shall be no penalty on the additional tax because of such understatement, but interest shall be added to the amount of the deficiency at a rate determined in accordance with § 58.1-15, from the time the return was required by law to be filed until paid. The penalty under this subsection shall not be applicable to any month or fraction thereof for which the corporation is subject to the penalty imposed under § 58.1-450. In no event shall the total amount of penalty assessed under this subsection and under § 58.1-450 exceed thirty percent in the aggregate.