Mark lets you mark invoices as Sent, quickly. Command-click or Shift-click to select Invoices you want to change. Then click Mark and select Sent.
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Pictured, some invoices were found and the Sent column displayed by clicking Add(+) and selecting Sent (now at the top).
Next, the invoices to change are selected by Command-click or Shift-click (for a range).
Then, click Mark and choose Sent to mark the selected invoices as sent.
Mark is a shortcut and works for locked and protected records. To remove Sent use Update to change Sent to No.