Perfectly normal. It would be inconceivable to an Italian that you wouldn't give your CF or codice fiscale to a vendor. It's 100% normal and it will be the post office or shipping company that insists on it. When I order from Amazon outside of Italy I am asked for my CF for customs clearance by Amazon. When DHL or any other courier is about to deliver from outside the EU, i will get atext, sms or email asking for my CF for customs clearance. As they know you are not resident in Italy and haven't got a CF, they ask for a passport number.
It's 100 normal here, It is also used in Italy as a way of keeping track of what a peson spends. You have to declare your income every year in Italy. It's quite normal for the guardia di finanza, or fiscal police to stop someone in a ferrari and find that he has no income. It's all about tax evasion here but in your case just about customs clearance.
I'm organising shipping a Jag V8 engine from Derbyshire to Italy for a friend. The first thing the UK shipper asked, apart from size and weight, was do the shipper and consignee both have registered EORI? Your EORI hereis your CF or vat number prefixed with IT.