Ticket Prices

Thank you for explaining the standard prices on here. Is it any more expensive if you decide to buy it from the train station on the same day you want to travel? I plan on traveling from Shanghai to Beijing next weekend, but I'm not yet sure if I will want to stay there for one night or two. It would be better if I could decide once I am there.

One other question: does the second class price guarantee you a seat for the entire journey?