Nothing in this part shall be construed to prohibit any common carrier from establishing and charging a lower than a maximum reasonable rate for the transportation of property when the needs of commerce or public interest require. However, no common carrier subject to the jurisdiction of the commission may establish a rate less than a maximum reasonable rate for the transportation of property for the purpose of meeting the competitive charges of other carriers or the cost of other means of transportation which is less than the charges of competing carriers or the cost of transportation which might be incurred through other means of

transportation, except upon such showing as is required by the commission and a finding by it that the rate is justified by transportation conditions. In determining the extent of such competition the commission shall make due and reasonable allowance for added or accessorial service performed by one carrier or agency of transportation which is not contemporaneously performed by the competing agency of transportation.