However, instead of paying for that bag outright, it's smarter to upgrade your ticket. You'll save around $11 per ticket by upgrading from the LowFare class to the LowFare+ class, which includes a complimentary checked bag and a carry-on. The LowFare+ class also includes a free seat reservation and meal.
The sale lasts from December 5 through December 7. That short time frame coupled with the limited tickets means you have to move quickly if you want to give winter the bird and head to warmer climes in 2018.