These custom fields will now be available for all the estimates/invoices that you create. However these will not appear on your Invoice PDF that your send. In the case of the above example, the custom field 'Delivery Date' will appear when you create a new invoice. Refer image below.
Please note: Custom Fields are available for you to store certain specific information about the invoice /estimate and it will not be displayed on the Invoice/ estimate PDF that you email or print.
Zoho Invoice FAQ