Your contract is your first line of defense when a client questions the finished product or the invoice. If the additional charges are addressed in the contract, simply pointing this out to the client may be enough to bring them around. It will also provide you with protection if your payment dispute has to be resolved by other means -- mediation or court. If your contract doesn't spell out how to handle changes in the project or if you were working without a contract, you'll have a more difficult time protecting yourself if a client wants to dispute your charges.
If you don't have a contract, I hope you kept a paper trail of all discussions and all changes. If you can show the client where you discussed how certain changes would make the project cost substantially more (such as more expensive paper or a more expensive printing technique) then you might be able to persuade the client to pay up. This documentation will also be helpful if you decide to use a third-party mediator or if the dispute lands you in court.
It's possible that you may end up having to take a loss on this particular project. If so, before you do another job for this or any other client -- get a contract. And make sure that contract addresses such common situations as revisions and substantial changes to the scope of the project.


