Southwest Airlines offers 15% off most roundtrip airfares when you pay via PayPal. That's the best percent-off coupon we've seen that's valid on any roundtrip purchase at Southwest. Sign up here to get your 15% off coupon code via email. The discount can be used on most fare levels and applies to travel booked by December 1 for travel by January 31. Some blackout dates and other exclusions apply.