I'm attempting to use gchecky and get_server_post_cart_url to post a cart
to GC.  There doesn't seem to be any method of requesting the url without
doing it myself.  _send_xml seems to be available (including Basic Auth)
but this uses a hard coded get_order_processing_url.

Is there some other method such that I can pass in an order to be delivered
to server_post_cart_url, and retrieve the response as a checkout_redirect_t?