Recently I traveled with American Airlines. I had a bad experience as I felt they were terribly outdated, so I complained with their customer service, who, ironically, gave me a paper voucher that I can redeem on my next flight.
Now... seems like this paper voucher that looks like a plane ticket can't be used while booking online, is that true? How I can redeem it? I read somewhere that I have to do a phone reservation and then mail the voucher. Really? That's so outdated
Then, call them and give the paper voucher code and the booking number that you will see in the next page. They will adjust the price for you to pay.