There may be situations where invoices (paid or unpaid) need to be deleted. A couple of steps are necessary to do so. 

1. Already paid invoices cannot be deleted until the underlying payment(s) are deleted first. Identify and delete all payment-type billing items (i.e. "Payment from Client" and "Payment from Retainer") applied to the invoice. This will change the status of the invoice from "Paid" to "Sent."
2. Invoices cannot be deleted until the status is "Pending." Open the invoice and change the Status to "Pending" and save it.
3. From the invoice summary screen, select the invoice and click the delete button. A popup window will indicate any potential problems preventing the invoice from being deleted. Resolve any issues and try again.

Note: If an invoice is unpaid, step 1 will be unnecessary.