Add Frontier to the growing list of airlines charging to cash in frequent flyer miles. Beginning Sept 15, they'll start charging $25 to cash in, and $75 if you do so 14 days or fewer before travel, for a total potential hit of $100. They're also upping the number of frequent flyer miles required on most itineraries by 5,000 miles. If you haven't already checked it out, see our handy airline frequent flyer fee chart.