If you are not using Styles for some other purpose, then you can color the customer based on their preferred send method.

Make it a five part form and set the invoice email for those customers who do not want emails to an email address that gets automatically deleted ("auto-delete@softrak.com"). Those customers who do not want the printed copy, have a field in their record that says "Recycle this page" and print that on the printed invoice in 72 point font.

However, why don't you want to use PrintBoss? This is exactly the kind of forms handling problem it was designed to solve.Your client will see an immediate return on the investment with fewer errors and simpler invoicing procedures.