Frontier Airlines takes 18% offnationwide fares via coupon code "BIRTHDAY18". (Click on "Book a Flight" to book; then click on "Promo Code?" to enter your promo code.) That's the best general-use percent-off discount we've ever seen from this airline. (We typically see 10% off.) Blackout dates and additional taxes and fees apply. Book by July 5 for travel July 24 through November 14.