Invoices can be deleted directly if they have not been sent to accounting yet.

If the invoice has been sent to accounting or skipped, the delete button will not be present.   If there is a scenario that you need to remove that invoice from a project, you are required to remove (delete or void) it from your accounting package first (WorkGuru cannot do this) and then back in WorkGuru, click on Queue Resend to Accounting.  This will re-enable the Edit and Delete buttons.