Tourist bus route 306 from Xian Railway Station

Hello, i want to start booking my holiday but i just want to check 1 detail first. Your sight says free shuttle bus from Xian Railway station if you buy the tickets form the tourist center at the railway station. If i already have the tickets before i arrive in China, can i still use the shuttle bus or not.

Also, how far before travel should i book my bullet train tickets.

Thank you