Frontier Airlines Book A Flight: As the eighth-largest commercial airline in the United States, Denver, Colorado-based Frontier Airlines is a well-known ultra-low-cost carrier with a vast network that includes nearly 100 domestic destinations and 30 international ones. Frontier’s operational culture is centred around the principle of “Low Fares done right,” which serves as the foundation for the company’s commitment to offering reasonably priced travel options without sacrificing quality.