You'll need to enter the addresses directly in the Ship To field, Clyde. I'll share further details about this process.
QuickBooks Online (QBO) is dependent on the setup of your Customers, Sub-customers, and Vendors' profiles. In this case, you'll need to manually enter the Ship To information to reflect your sub-customers address each time you create an Invoice or Purchase Order (PO).
Moreover, create custom fields to help you track your sub-customers and add a Bill To fields when creating PO transactions. Since we can only add the customer's address to the Ship To field. To use custom fields on purchase orders, turn on the purchase orders setting. Now, refer to the steps below on how to set this up:
- Navigate to the Settings menu and select Custom fields.

- Choose the Add custom field or Add field.
- Enter a name in the Name field.
- Click the Data type dropdown for the type of custom field.

- Select the category. (Customer, Transaction, Vendor, or Project)
- In Select Forms, choose the checkbox of the form/forms you want the custom field on.
- Once done, click Save.

I'll share this article for more info: Add custom fields to sales forms and purchase orders.
Lastly, I've included some helpful articles to guide you through managing customers and vendors within QBO:
As always, feel free to comment or Reply with any further questions about managing your customers, vendors, or sales activities. I'll be here to assist you and provide any help you need.