I want give you also a suggestion about another function that can be really good for the script.
It would be nice if the plugin send also the emails to the publishers when:
1) The invoice is released
2) A new advertising package is added
3) Before the services bought experied as:
10 days before,
7 days before,
3 days before,
and also last day and maybe some days after the service expires.
In this way they know that the service is near the expiration so they can renew in time and this is also good to don't loose the actual pubblishers, that, receiving the emails remember of it and is also a way to sponsor our sites remember them the name of the site also with the emails.
This I think could be really useful.