Does this $35 Southwest Airlines gift card have fees or an expiration date?

The gift card has no fees and does not expire.

How can I use this Southwest Airlines gift card to pay for a flight booked online?

After selecting flights and entering passenger information at southwest.com, choose “Apply Travel Funds,” enter the gift card number and PIN, and click “Apply Funds.” If the card does not cover the full total, use another form of payment for the remaining balance.