When will I know if you were able to buy my ticket and when do I pay?

I want to get a private soft sleeper from Xi'an to Beijing. I already know the date and train that I would like. I know tickets aren't available more than 20 days in advance, but can I order earlier with travelchinaguide so that when the tickets are released they can buy it as soon as possible? If I do that, when will I know if they were able to purchase the ticket and at what point do I have to pay?