"... Конечная цель - эти данные будут go в SQL таблицу ..."
Вы не указали поставщика базы данных, но если конечный пункт назначения - MS SQL Таблица сервера, вы можете загрузить XML напрямую, без преобразования XSLT. Проверьте это.
SQL
DECLARE @xml XML =
N'<ShipNotice PONumber="123456" DropShip="false" Branch="ACME" ShipNoticeId="987654321">
<Items>
<Item>
<Quantity>1</Quantity>
<UnitPrice>715</UnitPrice>
<SellerPartNumber>24ABB336ABN3</SellerPartNumber>
<Serialized>false</Serialized>
</Item>
</Items>
</ShipNotice>';
-- uncomment line below when you are ready
--INSERT INTO tbl (PONumber, ...)
SELECT c.value('@PONumber','INT') AS PONumber
, c.value('@DropShip','VARCHAR(5)') AS DropShip
, c.value('@Branch','VARCHAR(50)') AS Branch
, c.value('@ShipNoticeId','VARCHAR(50)') AS ShipNoticeId
, i.value('(Quantity/text())[1]','INT') AS Quantity
, i.value('(UnitPrice/text())[1]','DECIMAL(10,2)') AS UnitPrice
, i.value('(SellerPartNumber/text())[1]','VARCHAR(20)') AS SellerPartNumber
, i.value('(Serialized/text())[1]','VARCHAR(5)') AS Serialized
FROM @xml.nodes('/ShipNotice') AS t(c)
CROSS APPLY t.c.nodes('Items/Item') AS item(i);
Выход
+----------+----------+--------+--------------+----------+-----------+------------------+------------+
| PONumber | DropShip | Branch | ShipNoticeId | Quantity | UnitPrice | SellerPartNumber | Serialized |
+----------+----------+--------+--------------+----------+-----------+------------------+------------+
| 123456 | false | ACME | 987654321 | 1 | 715.00 | 24ABB336ABN3 | false |
+----------+----------+--------+--------------+----------+-----------+------------------+------------+