My sister lives in CA and a few months ago I bought a 10217 from Amazon.com and now it' in my sister home (Amazon.com doesn't ship lego to italy)

Which is the best/cheapest way to ship it to italy?

Use US Priority International. It is the best cost efficient way to ship with some degree of confidence. You can use USPS.ocm calc for quotes. Be sure she uses delivery signature confirmation if it is offered. Also, put "gift for my brother" on the customs form with a smaller $ declared value.

I mail sets to Italy and never had a prob. The last one was an Imperial Flagship and cost me like $51 to mail. I used 7 lbs.

Just be sure she buys the postage online since they charge much more at the post office. Use USPS.com to buy and print the shipping label. The customs form will automatically print with the label. The clear shipping bag (pouch) for the label can be picked up at any post office or ordered online or me (make sure to use these). https://store.usps.com/store/browse/productDetailSingleSku.jsp?categoryNav=false&navAction=jump&navCount=0&productId=P_FORM_2976_E&categoryId=subcatMSS_International

Thank you very much