Free Shuttle Bus to Terracotta Warriors Musuem

Hi! Am arriving at Xian North Station at 11:30. The free shuttle bus leaves at 12pm? Can I purchase the ticket online, show both the online ticket and railway ticket (from Beijing) when boarding the free shuttle bus?

Kindly advise if it's better to leave luggage at the free luggage storage at Terracotta (nearer to our hotel) or to leave it at Xian North (meaning i'll have to travel back and forth). My main concern is theft at Terracotta luggage storage.