What is Transactional vs marketing SMS?

The distinction is about content, not technology: both travel the same route at the same price. What changes is the legal footing. Marketing messages need consent and a working opt-out; a purely factual transactional message about an existing order generally does not.

Adding a promotional line to a transactional message reclassifies it. An order confirmation with a discount code attached is a marketing message and must meet those obligations.