Can we just come to your station and buy a ticket there and no need of having it reserve earlier and pay thru credit card?

Yes u can however it will be cheaper online

Yes you can but it will be more expensive. Also, if u purchase online....you'll need to do so at least 2 hours in advance of the departure time.

How much it will cost?

Sure you can do that .but it Will be more expensive

Yes but prices more than likely won't be the same as if u were to buy it online.