I called American Express Australia to report the defect & I was transferred through to the American call centre.
The CSR to whom I spoke transferred me through to a different department, after I explained that I didn't have an account. She did ask whether "I received an email" which I assume was some sort of inquiry as to whether I had been phished.
I then spoke to an online services rep., who after asking for my card number, listened to my report. She then put me on hold.
(The call had taken 10 minutes by this time).
After a few more minutes on hold, the CSR came back on the line, asked me to repeat the information, and confirmed for the umpteenth time that I don't have an American Express card. I explained that it wasn't my find, but that it had been published online & so was by now _very_ public.
(15 minutes by this time, most of that on hold listening to advertising for American Express, including some ironic praise for their website).
CSR comes back on the line. She's spoken to her 'technical team' who assure me that there's nothing insecure going on because it's all over HTTPS. So I politely walked her through the process - visit the page, add ?debug to the URL, click the admin link & behold: lots of should-be-secure stuff.
At this point she thanks me profusely, & asks that I hold while she speaks to her supervisor. Back to the American Express ads ...
(20 minutes at this point).
The CSR came back on the line, thanked me again, & said that her supervisor had taken a screenshot of the issue & escalated it. Job done.
So, yeah, I can totally understand the frustration experienced by the guy who discovered the vulnerability. But it certainly wasn't impossible for me to report the issue, & I'm in Australia.