(b) U.S.-flag air carrier service is considered available even though—

(1) Comparable or a different kind of service can be provided at less cost by a foreign-flag air carrier;

(2) Foreign-flag air carrier service is preferred by, or is more convenient for, the agency or traveler; or

(3) Service by a foreign-flag air carrier can be paid for in excess foreign currency (unless U.S.-flag air carriers decline to accept excess or near excess foreign currencies for transportation payable only out of such monies).