You can create an invoice from an estimate.

To do this, click the "Finance>Estimates" tabs.

Find the estimate you want to turn into an invoice and click the "Magnifying glass button" to edit.

Once on the Estimate page, click the "Create Invoice from Estimate" button at the top.

A pop-up will ask you if you want to turn this estimate into an invoice. Click "yes".

(Note: The invoice cannot be turned back into an estimate)

You will then see the invoice that was created from the estimate.