There is no difference in price for advance buying or just before your trip. Tickets for travel within Switzerland can be bought online at www.sbb.ch (not possible for tickets for trips between Italy and Switzerland).

Useful links:
http://mct.sbb.ch/mct/en/reiselust/int-gaeste.htm
http://www.swisstravelsystem.ch/?L=2

Depending on your travel schedule on Swiss trains, a Swiss Pass may offer you a great benefit.