Cant find any dates that the promo works
The "free" is just on the base fare (sample I'm looking at is $7 per direction).
You're still on the hook for:
9/11 fee of $5.6 each way.
Airport facility fee of $4.5 each way.
Online booking fee of $4-$21 per direction.
If you're a Discount Den member, you don't pay the Online booking fee. Otherwise, you can go book in the airport (same like Spirit) to avoid it.