if the itenary is A->B->C, since A->B is a segment and B->C is another segment, the requirement spec. already said that the total price is equal to the summation of indiviudual segment, so I think that 'flat price per destination' should be rephrased as 'flat price per segment'. If 'flat price per destination' implies that the price is the same for A->B->C and A->D->C, then the ticket of an itenary from A->B is not of the same price as the ticket which is a segment of another itenary A->B->C.
Imagine if the price is irregardless of the route from a point to a destination, then there is no need to do add the price for individual segment.