Sooo, I figured my problem out on my own! I never heard from Pro Series so called "customer service" and in the end, I'm glad I figured it out instead of waiting for them.

For some reason, I have to uncheck the box on the invoice that says to add a "page break" between the invoice and the forms you want displayed. When I uncheck that box, everything's back to normal. I get one letter and one invoice (with a blank page after the invoice)!