It is true that Purchase Orders cannot be customized in the same way that Invoices, Sales Receipts and Estimates can in QBO. But you can use the baby gear icon and add your own custom fields to a Purchase Order, and you can turn-on custom transaction numbers. Estimates are for Customers and Purchase Orders are for vendors. If you need to present an Estimate to your Customer for this Subcontracted Service then you would create an Estimate for them.