Providing the best experience with the product is what we aim for, @paulirwin. 

I know how beneficial it is to your business to add the Purchase Order (PO) number to your invoice. However, we can only add a custom field to your invoice to include your PO. With this said, I'd recommend sending this request straight to our product engineers through feedback. Sharing features and options that you would like implemented is how our engineers look for new product updates. Here's how: 

1. Go to the Gear icon at the top.
2. Select Feedback.
3. Enter your comments or product suggestions.
4. Then select Next to submit feedback.

Then, to add a custom field to your invoice to include your Purchase Order, follow the steps below: 

1. Click the Gear icon on the upper right corner.
2. Select Account and Settings.
3. Go to the Sales tab on the left menu.
4. Under Sales form content, click the pencil icon to add a custom field.
5. Tick the box and enter the PO number under the Name field.
6. Click Save and Done.

To get more details about the process, feel free to visit this article: How to add custom fields to sales forms and purchase orders. If you need to use custom fields on your reports, here are the steps you can follow:

1. Find and open the report you want to run.
2. Select the gear icon at the top right of the report.
3. Under Change columns, check the box for each custom field you want to add.
4. Select anywhere outside the settings panel to close it.

Customizing your invoice forms to reflect more of your business identity is easy with QBO. To get started, please head to this article: Customize invoices, estimates, and sales receipts in QuickBooks Online. This page will also guide you on how to upload a logo and reorder columns.

You can count on me if you ever need additional help with adding a custom field on invoices. I got your back. Have a great day ahead.