You could additionally test the three helper functions. But the original tests against ProcessOrder would still be needed for completeness, so they wouldn’t necessarily add much except in an Uncle Bob style, “He who dies with the largest burden of gratuitous micro-tests wins,” sort of way.
Now if I really wanted to make that code easier to test, I’d instead be looking into ways to make the whole thing less stateful. Temporal coupling is much more confusing than if statements.