You are unable to delete an invoice in E-Clinic if it has a charge payment or credit on it so firstly you should delete/remove, like in the image below.
Once this has been removed the invoice will look like the one in the image below. You can see there is no charge or payment on the invoice and so that is why the 'Delete Invoice' button is available. (this does not delete the charge just removes it from the invoice)
When you click 'Delete,' a warning message appears like the one in the following image. Click 'Yes' to delete or 'No' to cancel.
