You called Buster!
As you said, Amex fees have traditionally been higher than for other card based products although the acquisition market is now very competitive and some acquirers will take all of a merchants card based transactions in order to get the volume, including AMEX transactions.
LGS6753 is also spot on with his analysis of fee structures etc. Believe me, it's a tough market out there.
Accordingly, it's difficult to know whether Aer Arran's charges to passengers are "excessive" or not.
OLY1B