You can definitely create a Purchase order for multiple projects on the same Purchase Order, whiteghost. Let me provide the outlined steps below.

1. Go to + New, then select Purchase order.
2. Select the Vendor.
3. In the Item details section, enter the parts you want to purchase.
4. Choose the Project you want to link the item to.
   
5. Enter the amount.
6. Click Save and Close.

Once done, you can record it as a bill. Since it involves multiple projects, you need to remove the line when recording it to ensure separation. Here's how:

1. Open the Purchase order you created.
2. Click Copy to Bill.
   
3. Find the item you need to remove and click the trash bin icon.
4. Select Remove line.
   
5. Hit Save and close.

Furthermore, after making a payment for your vendor, you can use this link as a guide to record the bill as paid: Pay Bills.

Additionally, you'll need to create an invoice and credit memo to the sub customer since they will be the one who'll pay for the invoice. You can open these articles for references:

• Create invoices in QuickBooks Online
• Create and apply a credit memo

This ensures that your accounts reflect the flow of parts and repair transactions accurately across your books. Leave a comment below if you need further help with purchase orders and I'll be here to assist you.