I just uploaded a new version that I believe fixes the issue posted by BlindWolf above... if line items weren't getting added to an order early enough, the free order payment method would still be checking an order total over $0. To fix this, I've moved the check to ensure an order total of $0 to the review order form creation instead of the checkout form validation. It'll just kick you back to /cart/checkout if it thinks you got there inappropriately.

Let me know if this solves the problem and if it still works properly in other cases.