No l paid by Amex, no fee.

I was not informed of a fee if I was charged one. They simply advised of the total cost. I paid 50% when I booked through their website and the remaining 50% when I checked in. I declined to ask for a receipt so I honestly don't know if there was a fee.

I've checked the details of the confirmation email from Casa Balthazar : they just wrote they didn't work with American Express and kindly asked me to provide another credit card., which I did. But no extra fee was charged . Hope this is helpful and you'll enjoy your stay.