I know how it works for Triggered Sends -
and the notable difference between All Contacts / All Subscribers.
Question: Can the "new" transactional API used in a way that does not create billable contact records? Meaning that it could be used using external Ids for recipients (e.g. non-Salesforce Ids) paying only the Super Messages for the sendout, not creating additional billable Contacts that might have to be deleted. I couldn't find any reference in the documentation on that, and since Contact Count is not easy to determine in real-time, I don't trust my own tests from a while back.