如何更新已使用PHP工具包完成/计费的Netsuite事务

正如标题中所述，我试图更新Netsuite（本例中为销售订单）中交易的自定义字段，但遇到了无法找到解决方案的问题。现在，我的代码在尚未开票的销售订单上运行良好，但当SO开票时，我得到以下错误：

UpdateResponse Object
(
    [writeResponse] => WriteResponse Object
        (
            [status] => Status Object
                (
                    [statusDetail] => Array
                        (
                            [0] => StatusDetail Object
                                (
                                    [code] => 
                                    [message] => 
                                    [afterSubmitFailed] => 
                                    [type] => ERROR
                                )

                            [1] => StatusDetail Object
                                (
                                    [code] => WARNING
                                    [message] => Items on this line have been fulfilled and billed.  Are you sure you want to modify it?
                                    [afterSubmitFailed] => 
                                    [type] => WARN
                                )

                            [2] => StatusDetail Object
                                (
                                    [code] => WARNING
                                    [message] => Items on this line have been fulfilled and billed.  Are you sure you want to modify it?
                                    [afterSubmitFailed] => 
                                    [type] => WARN
                                )

                        )

                    [isSuccess] => 1
                )

            [baseRef] => RecordRef Object
                (
                    [internalId] => 74452024
                    [externalId] => 
                    [type] => salesOrder
                    [name] => 
                )

        )

)

我理解为什么我们不想编辑已经过账到总账的交易，但我尝试更新的字段不会影响总账；这只是一个自定义字段，我们将使用它来跟踪我们在线商店中的一些礼品卡信息。我可以在UI中编辑记录并在没有任何警告的情况下保存它，但是我不知道如何通过PHP工具包使其工作。非常感谢您的帮助。我的代码如下：

require_once ('netsuite/NetSuiteService.php');

$ns = new NetSuiteService();

$field = new StringCustomFieldRef();
$field->internalId = "custbody_bc_gift_card";
$field->value = "Test This";

$so = new SalesOrder();
$so->internalId = "xxxxxxxx";
$so->recType = new RecordRef();
$so->customFieldList = new CustomFieldList();
$so->customFieldList->customField = array($field);

$update = new UpdateRequest();
$update->record = $so;

$res = $ns->update($update);

print_r ($res);