Note: deliveries that are in use (configured on bill groups/accounts) cannot be deleted. In order to delete an invoice delivery that is in use, first check the References tab to determine which entities are configured to use the delivery you wish to delete and then change the bill group/accounts to use another invoice delivery.
Under the Invoice Deliveries heading on the left select the delivery item you wish to delete
Under the same Invoice Deliveries heading click the icon
On the confirmation dialog that appears select Yes