Credit card processing fee

I booked a car rental online and went to pick it up today. I took out additional insurance at the depot and was charged a credit charge processing fee - over €6.00 plus VAT for my trouble.

Can they do this?

So long as they make you aware of the charge they can. Ryanair do it all the time.

They didn't. They told me after I had signed the slip when I asked for the precise breakdown.