At the time of transmitting the return required under § 58.1-615, the dealer shall remit to the Tax Commissioner the amount of tax due after making appropriate adjustments for purchases returned, repossessions, and accounts uncollectible and charged off as provided in §§ 58.1-619, 58.1-620 and 58.1-621. The tax imposed by this chapter shall for each period become delinquent on the twenty-first day of the succeeding month if not paid.