Did they mail it and then charge you or did they return the one with the incorrect Post?
You may be able to make a deal with them that you put what post on it the computer says and if there is an overage they can put an envelope in your box with the total needed. Mine had done that before as well.
You may be able to make a deal with them that you put what post on it the computer says and if there is an overage they can put an envelope in your box with the total needed. Mine had done that before as well.