Here in the EU, credit card transaction fees are capped at 0,3%. So no shenanigans, but cryptocurrencies would have to fall within this range as well.
This isn’t true. Interchange fees are capped at that rate, but that’s only one portion of the total transactional fee charged to merchants for processing card payments.
This means that there's a chance your merchant has been paying more for each Amex payment.
[1] https://curia.europa.eu/jcms/upload/docs/application/pdf/201...