The Tax Commissioner may grant a reasonable extension of time for filing intangible returns whenever in his judgment good cause exists. Except in the case of a taxpayer who is abroad, no such extension shall be granted for more than six months. Whenever the time for filing a return is extended, interest at a rate determined in accordance with § 58.1-15, from the time the return was originally required to be filed to the time of payment, shall be charged and collected. If any taxpayer who has been granted an extension of time for filing his return fails to file his return within the extended time and to pay the full amount of the tax as shown on the face of the return at the time of filing, and the accrued interest, his case shall be treated the same as if no extension had been granted.