Select alternative custom or related email field for double opt in/invoice

The restriction of sending double opt-in and invoice email's to the default contact record email field is unecessarily restricting a range of business use cases.

There are many circumstances where the email for communication exchange and uniqueID is not the desirable email for sending invoices. (think of all the applications that allow a "billing email" field)

In addition, there are many cases where the contact for communcation is not the same person as the client's contact record. (think all services who deal with children, elderly or people with disabilities where the contact record for the "client" is not the contact record that needs to receieve correspondence.

Ontraport's custom objects and custom relationships open's amazing possibilities to create systems that handle these circumstances but hit a brick wall to either send invoices to the correct address (if it is not the default contact record email) or double opt in a realted contact who needs to receieve critical correspondence about the services of the client they are a carer for.

Just like in ontramail the setting used to exist on the invoice/double opt in template builder to send an invoice/double opt in to any email field within the contact record or any related object email field. However, it had apparently not been coded to function and was removed. 

Could the function be completed to further unlock the versitility of ontraport to handle complex client management circumstances.