Book quickly! Frontier has more $39 roundtrip fares valid for purchase today only! On top of that, there is also a promo code, which will take up to $20 off the roundtrip base fare ($10 each way).

Unfortunately, combining both of these sales will only bring the $39 roundtrip fares down to $37 roundtrip due to those fares already having incredibly low base fares. Frontier's promo code sales normally take a percentage off the base fare, which is great for higher fares, but the lower fares in the $70-$100 roundtrip range will benefit more with this straight $20 discount.

Both sales are valid today only and tickets must be booked by 11:59 p.m. ET, May 23.

Travel is valid on select dates through June 20. A 3-day advance purchase is required and blackout dates are listed for June 15 and 17. However, some routes may still be available on those dates.

Availability is limited so you must be somewhat flexible to find the lowest price. Travel days will vary by route. Many routes are not flown daily so be sure to click on the fare details for specifics. If you use the promo code listed below, you may be able to knock $1 off each way, though some routes may be excluded from the promo code sale.

A sample of nonstop $39 roundtrip fares include:

Denver to Las Vegas $39 roundtrip

Atlanta to Cincinnati $39 roundtrip

Atlanta to Orlando $39 roundtrip

Cincinnati to Raleigh $39 roundtrip

Buffalo to Raleigh $39 roundtrip

Detroit to Long Island-Islip $39 roundtrip

Philadelphia to Charleston $39 roundtrip

Long Island-Islip to Minneapolis $39 roundtrip

Omaha to Philadelphia $39 roundtrip

Denver to Palm Springs $39 roundtrip

Providence to Myrtle Beach $39 roundtrip

Trenton to Charleston $39 roundtrip

For the $20 off promo, just enter promo code 20OFF when booking on FlyFrontier.com to receive up to $20 off the roundtrip base fare ($10 each way) for nonstop domestic flights through June 20. Avoid blackout dates on May 25 and 28. If you want to piece together an itinerary flying only one-way on Frontier, you can use this promo code for $10 off one-way flights as well.

A sample of nonstop promo code fares include:

Philadelphia to Orlando $37 roundtrip

Denver to Jackson Hole $37 roundtrip

Charlotte to Orlando $47 roundtrip

Denver to Salt Lake City $49 roundtrip

Chicago to Charleston $49 roundtrip

Buffalo to Orlando $57 roundtrip

Providence to Orlando $57 roundtrip

Philadelphia to Tampa $57 roundtrip

Milwaukee to Austin $57 roundtrip

Raleigh to Milwaukee $57 roundtrip

Washington D.C. to Orlando $58 roundtrip

Long Island-Islip to West Palm Beach $67 roundtrip

Detroit to Austin $77 roundtrip

The discount does not apply to taxes nor Frontier's online booking fee, which they label a "Carrier Interface Charge". This means some routes may have less of a discount if the base fare is less than $10 each way. As always, Frontier has a long list of specific routes that are excluded from the promo code sale. 

These fares all work in reverse and there are many more routes priced under $99 round-trip as well. Check what's available from your city. Keep in mind that Frontier charges extra for any bags larger than a small personal item (small backpack, purse, etc.). Pack super light to get the lowest price. Checked bag fees start at $25 each way.

Above image by Colin A. Pierce via Shutterstock.