Voluntary compliance: paying assessed customs duty and interest precludes further notice and limits penalty exposure. A voluntary compliance scheme permits payment of customs duty or interest before notice, based on self-ascertainment or officer determination, and requires pre-notice consultation by the proper officer except in specified exceptions. Written notification of such payment bars further notice or penalty for that liability. Reduced-penalty treatment applies where duty or interest determined by the officer or specified in a notice is paid within the designated short period; full payment with penalty renders proceedings conclusive, whereas any shortfall permits the officer to issue a further notice within the statutory limitation period.