Which airlines offer a lowest airfare guarantee?

Two airlines that offer lowest airfare guarantees are American Airlines and United Airlines. Both airlines have a number of rules that restrict the ways in which a person can take advantage of these guarantees.

The American Airlines guarantee allows users to receive a refund of the difference in fare, plus a $50 promotional code for a future flight. However, this only applies if the buyer finds the same flight for a minimum of $5 less on another site, on the same purchase date.

The United Airlines plan is similar, but the same flight on another site must be $10 less than the price on United.com. If proof of the lower price is submitted to United, the airline offers a $100 voucher for future travel.