PayPal PLUS-在订单中添加发票编号

Question

我想将发票编号添加到PayPal PLUS订单中。我尝试了以下请求：

                                if($this->config->get('pp_plus_test') == 1)
                            {
                                $mode= ".sandbox";
                            } else {
                                $mode= "";
                            }


                            $fields2 = '[{
                              {
                                "op": "add",
                                "path": "/transactions/0/invoice_number",
                                "value": {
                                  "invoice_number": "125478"
                                }
                              }
                            }]';



                            if($this->config->get('pp_plus_test') == 1){
                                $mode= ".sandbox";
                            } else {
                                $mode= "";
                            }

                            $ch2 = curl_init();
                            $headers=array('Content-Type:application/json','Authorization:Bearer '.$this->getToken());
                            curl_setopt($ch2, CURLOPT_HTTPHEADER, $headers);
                            curl_setopt($ch2, CURLOPT_URL, "https://api".$mode.".paypal.com/v1/payments/payment/".$this->session->data['createPaymentArrID']);
                            curl_setopt($ch2, CURLOPT_HEADER, false);
                            curl_setopt($ch2, CURLOPT_SSL_VERIFYPEER, false);
                            curl_setopt($ch2, CURLOPT_CUSTOMREQUEST, 'PATCH');
                            curl_setopt($ch2, CURLOPT_RETURNTRANSFER, true);
                            curl_setopt($ch2, CURLOPT_POSTFIELDS, $fields2);
                            $result2 = curl_exec($ch2);

并获得此回复：

{"name":"MALFORMED_REQUEST","message":"Incoming JSON request does not map to API request","information_link":"https://developer.paypal.com/webapps/developer/docs/api/#MALFORMED_REQUEST","debug_id":"125482c756fe1"}

有人可以告诉我该请求有什么问题吗？