As a B2B SaaS owner, I don't accept Amex, because they make me jump through extra hoops. I process all cards through Braintree, but Amex requires an extra agreement directly with them, and they make the process of signing those agreements really difficult and obnoxious. I have better things to do with my time.
I did a quick search, here's a partial list of what was required:
· Please provide a scanned copy of Passports for authorized signers and Beneficial Owners.
· Please provide a scanned copy of Certificate of incorporation
· Please Provide a Completed Multi Currency form, Attached
· Please provide a completed and Signed Side Letter, Attached
· Please provide a completed and Signed W8 BEN form, Attached
(BTW, I am not based in the US, so "W8 BEN" reads like Klingon to me).