Invoice software to attach a PDF file to an e-mail and add text before it
Solution 1:
What you're describing is something that a batch emailer could probably accomplish for you (see software below).
Another option is to create a rule in Mail.app and attach an Applescript to it. The Applescript route would probably require some knowledge (and patience) on your part to effectively make your job easier.
- Mailings
- Direct Mail
- RoboPostman
- MailActOn