The mouseover pop-up for the CSPA charge in EVE mail settings does not detail how it works fully. It states that unknown characters not on your contact list will be charged a fee and that characters added to your contact list will not.
Currently, those in your corporation will not be charged a fee. Those in corporations in your alliance will. You can also corps and alliances to your contact list and all members will not be charged a fee. This behavior is not documented and does not match the text of the popup.
It would be great if the mouseover pop-up information could be rewritten to clarify how this feature works.