Hi Jean
Depending on the email program you use, you can schedule email deliveries. I use Outlook (2010), and it's just a case of creating a new email, selecting the options button, and setting the time you would like the email delivered. Click close, attach the invoice pdf, select recipient, and click send. It moves the email to the Outbox. When it reaches the time you've set, out it goes.
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Mike Smith
Viktorian Sheet Metal Mfg Inc