I'm not really sure, but I think you can't buy a SIM card before your departure.
I would recommend watching the folliwing link: https://www.tim.it/tim-visitors-en that might also fit your needs during your stay in Italy. Please remember that you can only buy a SIM card WITHOUT any plans in it only in Italy, not in advance.
You'll be able to get the sim easly at airport, activation is fast (usually less then 15').
You should also check if your phone supports the frequencies used by our carriers ( GSM: 900/1800, UMTS/3G: B1/B8 and LTE/4G: B3/B7/B20 ). Most important are bolded ones
If you plan to stay for more than a month, I strongly suggest you to buy a 'regoular' plan, with the addition of 'tim international per tutti' that lets you call the US for only 10c/min with 16c of connection fee.
PS: usually offers at 'street's corner' shops are not listed online and gives you a plenty of traffic for little money compared to online regoular offers (you'll be able to get up to 5 gigs and 1000 national minutes for about 10€)
also remember that, differently from NA, here RECIVING is free for both voice and sms
___________________________________________________________________ Tim Special per te + giga extra free