CSPA is a charge which CCP has put on all invites (convo or gang) and eve-mails to help prevent spammers.
I recommend removing it, as we fly a lot in gangs and have a lot of convos and probably also eve-mails - and it is annoying to have to accept payment of 1 to 100.000 ISK for each mail or convo or ganginvite. It is not so much the ISK as the annoyance of having to press OK one more time.
You risk that your fleet commander will refuse to invite you to gang if you keep CSPA charges - this is the case for all Agony gangs fx.
I have flown with CSPA charge removed for 3 months now and have no bigger influx of spam eve-mails than before. My advice is to petition, petition and petition spammers and ISK-sellers.
To remove CSPA, open your Eve-mail, rightclick Inbox and choose Settings.
Another option is to whitelist certain users. All members of your addressbook are not charged. So if you want to keep your CSPA charge for strangers, you need to add the following to your allowed list:
Again go into your inbox, this time go to the Allowed tab in the Settings. Now use the button to search for the corps and alliances listed above one by one and add them to allowed.