This has been touched upon in another thread but was incidental to the main discussion so I thought I would start a new thread on it.

I have a Euro-denominated prepaid fairfx Mastercard which I have used twice to buy UK-originating Ryanair tickets.
The exchange rate seemed rather high the first time round and I was surprised to see the converted Euro amount on the confirmation page but I ignored it.
The latest one a few days ago resulted in the same phenomenon, so I checked: an exchange rate which was some 5% above the interbank rate on oanda.com was used. Now, I doubt that the wholesale Mastercard loading is anywhere near that rate and my understanding is that fairfx do not charge anything themselves above the MC wholesale rate for purchases in a currency other than that of the denomination of the card.
Moreover, the converted cost appears on the Ryanair site itself on the confirmation page, which suggests to me that this is a dynamic currency conversion.
I do not recall seeing anywhere an indication that dynamic currency conversion would be used nor, a fortiori, an opportunity to opt out of it.
It is my understanding that dynamic currency conversion is only allowable if the customer expressly agrees to it.
So I wondered what other people's experiences were with this and whether they confimed my suspicion of clandestine dynamic currency conversion and, if so, how should one go about challenging it?