PS website says (Google translate):
The ground handling package is mandatory, pricing details viewable in price guide :
And the guide says 1-5 tonnes (bearing in mind this is a PA28 with MAUW 2450 lbs) is €46 plus VAT.
So either that's changed, or they don't mean it!
But it does say in that guide 'Handling is mandatory for any aircraft using the airport for a stopover.' Did you leave the airport? (I was planning on going off site for lunch - is that a stopover?)