If a test user leaves the site in a state where this will occur, we can certainly score the task as a success. If the user fails to place any order, we can just as easily determine the task a failure. But there are other possibilities as well. If a user does not perform a task as specified, you could be strict and score it as a failure. No middle ground.
Success is success, without qualification.
However, I often grant partial credit for a partially successful task. How to score partial success depends on the magnitude of user error. Of course, the precise numbers would depend on a domain analysis. There is no firm rule for assigning credit for partial success. Partial scores are only estimates, but they still provide a more realistic impression of design quality than an absolute approach to success and failure.