Search Southwest and Allegiant separately

These low cost leaders do not share their lowest fares with third party web sites, such as online travel agencies or meta search engines. (They're also frequent issuers of promo codes, which are redeemable only on their sites.) Southwest doesn't always have the lowest price, but in many markets they're the only airline flying a route nonstop, even if just once a day. Plus, they have the lowest extra fees.