Do I have to include extra luggage when I buy my ticket or can I pay when I check in my luggage?

Depends on the airline

You can pay when you get to the airport... I never prepay

Yeah deoends on the airline..

You can pay when you check in your luggage.. however as Kelita said.. probably best to call the airlines to see what their specific rules are regarding luggage. There seems to be some new guideline every so often.. and every airline is different.

You can pay when you check your luggage in. It might be a good idea to call the airlines that you're flying so you can get everything in order before you get there.