Ultra-low cost carrier Frontier Airlines is adding two more routes out of Atlanta, launching flights to San Antonio and Pittsburgh.

The Atlanta-San Antonio flights starts May 13 with introductory one-way fares of $59, while the Atlanta-Pittsburgh flights start June 17 with introductory one-way fares of $39. Both routes will operate four days a week. Various restrictions apply to the introductory fares.

Denver-based Frontier charges extra to bring a roll-a-board as a carry-on, as well as charging for checked bags. It also charges for soft drinks.

The airline sells a refundable fare bundle for a higher price -- $132 for Atlanta-San Antonio and $112 for Atlanta-Pittsburgh -- that includes a carry-on bag, a checked bag and other benefits like priority boarding.

FILE - In this Oct. 15, 2014 file photo, a Frontier Airlines employee directs passengers at Cleveland Hopkins International Airport in Cleveland. There are few businesses that consumers love to hate more than airlines, but travelers seem to reserve a special level of vitriol for the no-frills, discount airlines. (AP Photo/Tony Dejak, File)

Credit: Kelly Yamanouchi

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Credit: Kelly Yamanouchi

The new routes come a week after Frontier announced new flights to nine cities from Atlanta starting in the spring.

** FILE ** In this May, 24, 2006 file photo, a long line of Frontier Airlines Airbus 319's sit at the gates on concourse A at Denver International Airport. While past recessions have spawned ultra-cheap airlines, experts say the triple-digit oil prices that are pushing air travel out of reach for average Americans also will almost certainly keep a no-frills carrier from entering the market soon. (AP Photo/David Zalubowski, File)

Credit: Kelly Yamanouchi

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Credit: Kelly Yamanouchi