Duty payment timing requires monthly duty by fifth day, with intimation, interest on late payment and pro rata recalculation. Monthly duty for notified packing machines is payable by the fifth day of the month with prescribed intimation; late payment incurs interest under section 11AA. Differential duty from added machines, changes in retail sale price, or duty rate revisions is recalculated pro rata and must be paid by the fifth day of the next month, with refunds of any excess by the twentieth. Operation of machines producing undeclared retail sale prices triggers application of the highest applicable duty across all machines. Continued operation without payment incurs duty based on the higher of last declared machines paid for or machines found on premises.