View the invoice. This may be done at multiple locations. See View an Invoice.
Select the check box in front of "Invoice is marked to be Printed" or "Invoice is marked to be Emailed'".
Click Close. The invoice will now appear on the "Deliver Customer Invoices" list with a check mark indicating the need to be printed/emailed. It will also appear on the Print or Email All Undelivered Invoices report.