[Nursie]

Historically, Amex always required a separate retailer relationship and to act as its own acquirer. I don't know how true that is any more. They've just always been the awkward one, with higher fees and special relationships. Also they used to use ANSI standards on some stuff where everyone else used ISO... but that's going back 20 years!

[inkyoto]

Yes, that is still true – AmEx own their own payment processing network, and they do not allow outsiders into it, even banks they have the brand sharing agreements with, hence a separate retailer relationship.