I have no idea what to put it in and what happens when i take it to the post office do they say how much it'll cost any help would be good!

I have made a request for a swap and three have been made for my books. However when I sent the request it said make sure the list is up to date as they can choose any book at any time and its a legally binding agreement <<scared>>. Does this mean I have to wait until the person I made the...