I've updated the form that we used for creating invoices. It's now a google document and can be found here:
GDrive
Podio includes some explanation and link to understand Credit Notes better.
The file has three tabs now. One to use for Invoices, one to use for Cancellations, and one to use for Credit Notes.
Let me know if you have any questions. I'll make a time to go over this with each team so it's clear.