The increase in the tax shall apply to all unused stamps, meter impressions, and packages of stamped cigarettes owned by stamping agents at 12:01 a.m. on the day the increase becomes operative. On the date any change in the tax takes effect, each stamping agent shall take an inventory of all unused stamps, meter impressions, and packages of stamped cigarettes owned by the cigarette wholesaler at 12:01 a.m. The additional tax shall be remitted with the return for the last month preceding the date any change in the tax takes effect. The Tax Commissioner shall credit to each stamping agent an amount equal to the additional tax on two weeks of such stamping agent's average purchases of stamps.