Some people are receiving a link to register for 5000 miles for your next flight, booked and flown between 5/16-5/31. You can register below I believe:

https://www.flyfrontier.com/5000-travel-miles/

Reading the requirements for the For Less Guarantee, it seems almost useless in practice. For example, to use the guarantee for an Economy booking, the competing flight must be on the same dates/similar times, include a carry-on, seat selection, zone 1 boarding, and no change/cancel fee.