How do I apply this gift card to a flight booking on southwest.com, and what happens if it does not cover the full total?

After selecting your flights and entering passenger information, click “Apply Travel Funds” in the payment section. Enter the gift card number and PIN in the designated fields, then click “Apply Funds.” The gift card amount will be deducted from the total; use another form of payment for any remaining balance.

Where can I use this Southwest Airlines gift card, and what information do I need to redeem it?

Print the page with the gift card number and security code (PIN). You can use them at southwest.com, at a Southwest Airlines ticket counter, or by calling Reservations at 1-800-I-FLY-SWA.