Buying tickets at the station&baggage allowance on G type

We r planning to use bullet G type to travel fron Beijing to Shanghai.. It's always said that each person can take 1 luggage 20kg.. But what about the 7kg handbag? Do i have to pay extra for it?
And also, we r planning to be there end of October n thinking about purchasing the 2nd seat ticket at the station when we get to Beijing, so around 2-3days before our plan to go to Shanghai.. Is that possible? Coz I checked there r more than 15 G trains per day for that route, so is it really necessary to purchase online in advance? Especially around that time it's not actually high season, right?
Thank you..