Hi Leo,
If I understand correctly, your requirement is to exchange billing message with one of your customer? If you need to exchange same message with multiple customers, what will be the number of customers?
The message exchange pattern is B2B communication. However, it does not mean that you need to use EDI structure (unless this is a requirement) and B2B Add On. It could be a simple message exchange between your customers. It could be a custom message structure, IDoc, etc.
If you could elaborate your requirement, you will get more options.
Good luck.
Prabhu.