Attach documents to emailed invoice

We'd like to attach a set of documents to an invoice that is emailed, faxed or printed based on the items included in that invoice (e.g. License information, Terms of use etc).

I've created some relationships that would allow me to lookup documents per Item in an invoice. Now the question is how do I attach these at send/print time?

I hope I've expressed this clearly enough. Let me know if not.

Thanks,
D.