When such annual report for any year is not rendered to the Department of Public Service and the tax due thereon is not paid on or before April 15 next following, there shall be added to the tax an additional amount equal to five percent thereof or  $1.00, whichever is greater, if such return is made and tax paid within 15 days after becoming due, and 25 percent of the tax or $10.00, whichever is greater, if such return is not made and tax paid within 15 days after becoming due. When a company, which has failed to file such return or has filed an incorrect or insufficient return and has been notified by the Department of its delinquency refuses or neglects within 20 days after such notice to file a proper return, or files a fraudulent return, the Department shall determine the tax due according to its best information and belief and shall increase the amount of the tax so determined by 50 percent or $20.00, whichever is greater. No assessment shall be made under this section unless made within two years from the date on which a correct return should have been filed but the limitation of two years to the assessment of such tax or additional tax shall not apply to the assessment of additional taxes upon fraudulent returns. In its discretion, the Department may waive the penalties mentioned in this section if it is satisfied that the default was for any justifiable cause, and it may extend the time for filing returns or paying such tax, not to exceed two months. (Amended 1959, No. 329 (Adj. Sess.), § 39(b), eff. March 1, 1961; 1979, No. 204 (Adj. Sess.), § 12, eff. Feb. 1, 1981.)