Guangzhou to Hong Kong in late December

Hi, my first stop will be in Guangzhou and I am intending to take the intercity MTR to HK. However, I would like to buy the tickets online and collect them myself before travel but this option was not available..

Hence, I am wondering if it is possible to buy the tickets on the day I am traveling to and collect at the GZ East station? I did not want to a delivery as the delivery fees are very expensive.

As I will be traveling in late December, nearing the countdown, will the tickets be sold out?