The 30 Euros is a call out fee,however, it is charged per aircraft, not per call out. My understanding is that in order to be allowed to own and operate a refueling truck it is necessary to have an AOC. In FTE's case it would be complicated by the need to have two vehicles - one for Avgas and the other for Jet A1.
I know its per aircraft, hence the reason that we leave the refuel tag on the prop. Whenever the fueler is passing, he sees the tag and fills the aircraft. It is rare for us to ever need to call out the fueler to the piston aircraft. Saves a fortune in callout fees. If FTE were smart they could reduce the bill by adopting the same system.
When I need the Dornier filling with Jet A1 then I call them out unless I happen to be stood on the pan when he is passing from filling one of the 42's. But on a couple of tons of fuel €30 gets lost in the bill!! We do however have the same tag for it which I use if I have more time for refuel.
It is not an AOC that is needed for the fuel truck it is however an airside operators certificate for a fuel truck that is needed. You are right there is no chance that any of the based operators would be allowed there own installation. The Spanish are congenitally lazy but they are not stupid.