Frontier Airlines will send a $25 rebate card to travelers who purchase a flight at FlyFrontier.com and pay with any MasterCard. Purchase a domestic flight at FlyFrontier.com using promotion code MCBAG by Dec. 31, 2013 for travel through March 23, 2014. A $25 rebate card will be mailed to you within four to eight weeks after you fly your first flight segment. Quantities are limited to 4,000 rebate cards and certain flights or days of travel may be excluded from the offer. Only one rebate card is allowed per household. Visit http://www.FlyFrontier.com/MCBAG for more information.

Bottom line: Travelers flying on Basic or Economy fares are charged $20 at online check-in or $25 at airport check-in to bring one bag. If you check a bag, the $25 rebate will cover the price to transport your bag one-way but you will still be paying for the return trip. If you are able to travel without checked bags, you can essentially get a $25 flight discount.