You can now set whether or not a trigger can be used by merchants for transactional messaging by passing
transactional_allowed in the v1 Trigger API Endpoints.
Note: this requires the shop has transactional messaging enabled. It also does not mean that messages will be transactional for this trigger, only that the merchant has the option to enable these types of messages based off the trigger. The merchant has the control over this in the Postscript dashboard.