145. (1) The rate of duty, if any, applicable to any goods imported by post shall be the rate in force on the date on which the postal authorities present to the appropriate officer the declaration or label referred to in section 144 for the purpose of assessing the duty thereon. (2) The rate of duty, if any, applicable to any goods exported by post shall be the rate in force on the date on which the exporter delivers such goods to the postal authorities for exportation.