While not properly documented, the subscription calculator was never intended to be used to list subscriptions. We changed that recently, and it only accepts POSTs.
The body of the POST should be identical to that of the subscription, you can use the example from the subscriptions. I'll make sure to update the docs for the calculator, as the response has fewer fields (i.e. no URI, no ID and some more stuff can't remember right now). --- Reply to this email directly or view it on GitHub: https://github.com/jclouds/jclouds-labs/pull/38#issuecomment-29076882
