Several methods in the Card API are asynchronous only. These methods are asynchronous when Google anticipates some Payment Integrators will require the following sequence (e.g. due to batch files).
What if a Payment Integrator requires synchronous processing?
Rather than support asynchronous and synchronous versions of each method, Google has opted to only support one version per method. This is intended to make the launch of new Payment Integrators and ongoing maintenance simpler.
If a Payment Integrator only supports synchronous processing, they can implement the following sequence. Specifically, they can wait to return ACKNOWLEDGED until after that have successfully called the fooResultNotification endpoint.