WP-Invoice lets WordPress blog owners send itemized invoices to their clients. Ideal for web developers, SEO consultants, general contractors, or anyone with a WordPress blog and clients to bill. The plugin ties into WP’s user management database to keep track of your clients and their information.
With this plugin you will be able to send beautiful and styled html and plain text mails based on dedicated themes and templates for any e-mail notification issued for
* comments subscribers,
* your periodic newsletters or post notification (per post/daily/weekly/monthly),
* specific admin events : registration of a new user, comment to moderate, new comment on your posts.