I've been trying to buy Eurostar train tickets for a Rome to Venice round trip in late January (Mon-Tues travel)on the Trenitalia web site. After literally dozens of tries, I now see that the amica fare appears to be unavailable. I already have a prepaid reservation in Venice for an overnight and flying is not an option. What can I expect if I buy after we arrive in Italy? Am I going to pay much higher if I can only purchase a 1-2 days in advance?