It can be very time consuming to print, stuff and mail invoices so many larger clients will outsource that project to a 3rd party. See documentation for return on investment. 

This is a process to batch print invoices to a folder as a single pdf , which the client will review and forward to a 3rd party mailing house. This is for clients who want to send out new invoices for the day via snail mail and are using a 3rd party vendor who will burst and stuff and postmark the invoices and mail using bulk rates.  

NOTE: We think you should be emailing all invoices using ID800.