Do I need Customs clearance to fly an N-registered aircraft from Italy to Switzerland?
It's true. You need to take off from a authorised Airport with customs in Italy and land at an airport with customs in Switzerland as your first stop. 

Here is all information you need including a list of airports with customs. 
https://www.ezv.admin.ch/ezv/en/home/information-private/waren-anmelden/einfuhr-in-die-schweiz/warenanmeldung/grenzueberschreitende-fluege.html