U.S. citizens need a valid passport (that is not due to expire in the next six months) to enter Italy. U.S. citizens may stay in Italy up to 90 days without needing a special visa.
According to U.S. Customs and Border Control, an American citizen returning to the U.S. from Italy is “entitled to an $800 duty-free exemption and the next $1,000 worth of the goods you purchased is subject to a flat rate of 3%. If the value exceeds $1,800, the remaining duty will be determined based on duty rates in the harmonized tariff schedule, which are generally between 0-10% (except for clothing and textiles, which can be much higher, up to 25%).”
