With each customer record, you need to setup the invoice delivery method.  By default the Print Invoices box is checked.   To setup the Invoice Delivery method for a customer, open the customer record and click on the Misc. Customer Setting Tab.

Now check mark the invoice delivery method you want to use.

Invoice Only if ordered is delivered, shipped or picked up - Check mark this option to require an order be marked picked up, delivered or shipped before invoicing for that order will occur. NOTE: If this option is check marked and an employee or associate hands product to the customer, delivers product and/or ships product and never marks the order shipped, delivered or picked up, then the order will never get invoiced until that occurs. Use this option with caution.

Print Invoices – When an order is completed, an invoice is created and marked for printing.

E-mail Invoices – When an order is completed, an invoice is created and marked for e-mailing. (Note, when you check mark the E-Mail Invoice box another box will appear for you to enter a e-mail address to e-mail the invoice too)