If you do not wish to charge the non-recurring addon immediately, you can use the Charge Addon at Term End API. This will charge the addon during the renewal.
Please let me know if you need any further clarification.
Can we add just Unbilled Charge as we can do it in admin page?
The Charge Addon at Term End API will indeed create an unbilled charge for the non-recurring Addon. The API will only return an estimate of the Invoice Charges that will be raised during the subsequent subscription renewal unless you raise an invoice for it manually.
Hey, is there a way to create unbilled charge for existing subscription?
I need to create unbilled charge like this(but unbilled charge):