With regard to the mail merge field. Is there any mail merge available to show "estimated charge on renewal"
The smart section doesn't show this option for the other emails. How to actually get this?
The estimated charge on the next renewal is available only as a card field. It pulls data from multiple sources to provide a summary of the estimate - it cannot be provided as a mergevar.
Also, please note that this card field is only exposed to the following notifications: