Originally Posted by
invisible
That's because AMEX always handles currency conversion on its own and does not allow third parties to do it.
This is not universally true. I just purchased something yesterday from a foreign website that charges in foreign currency and advertises that they take American Express. The payment chain went like this:
Website -> 2Checkout -> Paypal -> AMEX
The transaction was converted to USD at the 2Checkout stage (they have a bunch currency options EXCEPT the local currency of the website; so it seems like they force you to use their currency conversion no matter what, and the rate is bad).