On AA and on US separate times, I got to the airport without any tix becasue I thought I had e-tickets but discovered at check-in that they had mailed me paper tix which I hadn't received or hadn't noticed.
They quickly filled out a "lost ticket affidavit" and sent me on my way, no processing fees and no hassle.
They did warn me if I found the tix and used them (as if I would) that I would be liable for full fares.