Ciao Nico,
I shared the computation replying to some other post in the HOC forum somewhere,
anyways, the amount onto wich the taxes and customs are computed is given by: declared cost of the goods + cost of shipping + cost of insurance(if any)
you will have to pay: customs + taxes + some other legal one-time importing docs
last time I could extimate around 36% added to original price
you might want to have a look at:
https://www.adm.gov.it/portale/dogane/cittadino/acquisti-su-internet you'll find your way around it
you might want to consider buying from ConEU too, that'd mean only: cost of goods + shipping (in Italy there are good Heritage guitars on the used market)
I think you get the picture ?