Notwithstanding the provisions of §§ 60.2-500 through 60.2-537, if on July 31 of any year the taxes or any portion thereof or the interest due thereon for any previous quarter is delinquent and unpaid and has been delinquent and unpaid for ninety days or more, the Commission may issue a notice of delinquency demanding payment. If the amount due is not paid within thirty days after such notice is mailed to the delinquent employer at his last known address, such delinquent employer’s rate for the calendar year immediately following the calendar year in which such notice is sent shall not be computed under the provisions of this article, but shall be 6.2 percent.