No common carrier or restricted common carrier by aircraft shall deliver or relinquish possession at destination of any freight transported by it until all tariff rates and charges thereon have been paid, except under such rules and regulations as the Commission may from time to time prescribe to govern the settlement of all such rates and charges, including rules and regulations for weekly or monthly settlement, and to prevent unjust discrimination or undue preference or prejudice; provided, that the provisions of this section shall not be construed to prohibit any such carrier from extending credit in connection with rates and charges on freight transported for the United States, for any department, bureau, or agency thereof, or for any state or territory or political subdivision thereof, or for the District of Columbia.