Current paperless office does not allow a different message per customer. You'd have to use something like KnowledgeSync (Sage Alerts and Workflow) and store the message you want to send at the customer level (e.g. in a UDF). Then you could have a different message per customer. KnowlegeSync can send PDF email attachments, so you'd have to have it attach that specific file name (which is a formula based on the customer and invoice number).

I have not used it before, but you might want to take a look at this enhancement (because it allows you to use AR_Customer fields, not just the defaults allowed by Sage... although I am not sure if UDF's could be used):

http://wiki.mbabsi.com/doku.php/enhancements/sage100/paperless/cs100p-merge

Kevin, 

Yes, a UDF could be created on the customer master file and reference that UDF using the Paperless Merge Enhancement. Paperless would insert the value of the UDF at the time of sending the emails.  

Knowledge Sync would also be able to do this as well. You can have two different events, one to exclude the special customer from the list and the other event would only send it to the special customer. Using a UDF like the Paperless Merge Enhancement can also be done in Knowledge Sync.