public function testCharge()
 {
     $client = new Client($this->configParams);
     $apiCallParams = array('amazon_reference_id' => 'S01-TEST');
     try {
         $client = new Client($this->configParams);
         $apiCallParams = array('amazon_reference_id' => '');
         $client->charge($apiCallParams);
     } catch (\Exception $expected) {
         $this->assertRegExp('/key amazon_order_reference_id or amazon_billing_agreement_id is null and is a required parameter./i', strval($expected));
     }
     try {
         $client = new Client($this->configParams);
         $apiCallParams = array('amazon_reference_id' => 'T01');
         $client->charge($apiCallParams);
     } catch (\Exception $expected) {
         $this->assertRegExp('/Invalid Amazon Reference ID./i', strval($expected));
     }
 }