The following is the only valid procedure to get them.
1. Purchase your tickets on Viator website
2. After the booking, check the inbox: you will receive a PDF voucher. Tickets booked after the office’s closure will be delivered within 8,30 am of the next day.
USE THE PDF SENT IN YOUR MAIL TO GET YOUR TICKETS, DON'T NEED TO PRINT IT
3. Go at the ticket office, ONLINE TICKETS WINDOW, at Viale Douhet, 2/a
4. Show the PDF on the screen of your phone and redeem your paper tickets.