This question is rather vague, so the answer cannot be precise, but I can suggest that you rewrite your billing application so that the output for each client is defined by a configuration file specific for each client.

Thus, every time a client wants something different, you haul out your configuration file and start tinkering. After a while you'll have a variety of formats and you can start with a solution that's close to the one your client wants, and go from there.