Book car hire now. you'll struggle (read:more chance of winning lottery) to book one there and then.No Joke
You only need to buy this is you cross the border on the main road and you buy it on the border crossing. Its 30 euro's.If you want to avoid paying this go the splugen pass way. ie through lecco and over the mountains.It takes approximately 20 minutes longer this way and is all hairpin bends. Some good scenery as well