I purchased tickets Florence-Venice-Florence, selected my seats, and received an email e-ticket instantly. It is my understanding that I just need to board and show the train staff the eticket when asked.
I will say the Trenitalia site is quite fickle. Aside from sometimes routing you back to the home screen, or taking you so far and freezing up, I was finally able to purchase on August 6 2014 for October 2014 travel and found the super econo fares for 19euro each way!!!! So I committed to the dates on my schedule to get these great, low fares. I went back into the booking routine and low and behold, it would let me select the econo (for 2 people) for the leg to Venice, and though the econo fares were showing in the selection screen as "from ... (amount)", each time I selected the super econo (not showing as sold out or limited seats) it said not enough available, even the next 2 price tiers up were coming back as not enough available, except first class -- and ridiculously priced.
Having just checked earlier that day and in the few days prior and seeing the low econo fares still available, I thought that really strange but tried the round trip again. I got the cheap seats to Venice, but to my amazement, the site came back saying that now there was nothing available for the date/time I wanted to return, not even the first class, nor was the one-hour before I wanted to leave available!! So, my cynical self thought, they must be tracking me. I cleared all browsing and cookies and reopened two Trenitalia screens, purchasing the one-way to Venice, and a one-way back to Florence -- both legs for the super econo fare, AND the date and AND time I wanted and had previously checked!! If you experience this same thing, try clearing all your cookies/browsing, as I do believe they track, and the system might not give you the best fares -- just my skepticism, but it was very odd.