cXML PunchOut session — edit your cart, then check out to return the order to the buyer.
Standalone test mode — no punchout setup session, so a test BuyerCookie is used and checkout posts to the fallback return URL
(https://punchout.mocked.site/return.php). Start a real session by POSTing a PunchOutSetupRequest to https://punchout.mocked.site/setup.