fix: Remove shipping method lookup

The plugin only uses the shipping method handle when generating the XML
document. This change removes the shipping method lookup and instead
sets the field value to whatever shipping method handle is saved for
the order.

src/services/Xml.php

@@ -150,8 +150,8 @@ public function shippingMethod(\SimpleXMLElement $order_xml, $order): void
         ]);
         Event::trigger(static::class, self::ORDER_FIELD_EVENT, $orderFieldEvent);
 
-        if (!$orderFieldEvent->value && $shippingMethod = $order->getShippingMethod()) {
-            $orderFieldEvent->value = $shippingMethod->handle;
+        if (!$orderFieldEvent->value) {
+            $orderFieldEvent->value = $order->shippingMethodHandle;
         }
 
         $this->addChildWithCDATA($order_xml, 'ShippingMethod', $orderFieldEvent->value);