how to email monthly bills automatically

Fellow NS users,

I sincerely hope someone can give me some advice on the following:

My Company sells subscriptions.
I have set up billing schedules to bill monthly over the course of 1 year.

For each customer I want to indicate whether they want to receive a paper
bill or an email containing their bill. We charge $2 for each paper bill, while emailed bills are free of charge. (our attempt to encourage our customers to think 'green')

Then for the customers who want to receive their monthly bills by email,
I would like NetSuite to send out the emails automatically based ofcourse