Yes, yes, I take all your remarks about my choices.
I am just a little surprised with the incoherency of these fees:
The ticket I was questioning first was Ryanair CPH-PRG. Card processing fee of 2,35 % was added.
Last night I purchased BUD-PRG also with Ryanair and paid with the same AMEX card. Surprise, surprise, no added card processing fee!
So, I still do not understand the pattern.