Southwest is the best for it, they even give vouchers for simply being delayed some of the time. United has been the worst for me as they are super cheap about it. I’ve actually used the paper vouchers from American before, however, I found the exact flights I wanted and then went to my local airport which isn’t too far from me and gave them the flight #’s that I wanted to book. They booked me and I was able to redeem the vouchers there. Still wasn’t as easy as booking online but I was worried about mailing them. It simply cost me an extra trip to the airport.