Troubleshooting How do I determine my customers who want me to mail them an invoice?
You could create a Customer Information Tag; Company > Group > Tags and tag those customers that need an invoice mailed. You can then run a report using your Job Tag Report to see each month who needs to receive a mailed invoice. Reports > Customer Reports > Job Tag Report.
REMINDER -- From the Customer's Portal they have access to a PDF copy of their invoices and could print them out themselves!