The taxpayer making the return hereinbefore provided for shall, at the time of making such return, pay to the city treasurer the amount of tax herein imposed; provided, that in connection with any return, the taxpayer may, if he/she so elects report and pay an amount based upon his/her total billings of business subject to the tax during the period for which the return is made exclusive of any amount previously billed, with prompt adjustments of later payments based upon any difference between such billings and the taxable gross receipts. (1997 Code)