Links a return shipping label to a return id. You can only create one return label per return id.
Since the label is sent to the buyer, the linked return label cannot be updated or deleted. If
you try to create multiple return shipping labels for a single return id, every create request
except the first will fail.
Create a request for the method "labels.create".
This request holds the parameters needed by the content server. After setting any optional
parameters, call the
AbstractGoogleClientRequest.execute() method to invoke the remote operation.