Thanks, glboisvert. I just tried and saw what you mentioned (before pressing 'purchase now', Roger: immediately after entering the cc number in the relevant box) when using a French visa card.
OTOH, when I use my Euro-denominated fairfx card, the amount in euros and the more info box do not appear, thereby giving me no opportunity to opt out. It seems that the script on FR booking page does not immediately recognise it as a Euro card when entering details (therefore not offering the additional info) but does recognise as such when actually proceeding to payment.