How can I add the customer P/O number on my invoices please. I'm on the basic package but surely this is a fundamental necessity on any invoice?

You can create a custom field to add your customer's purchase order number to your invoices, Paul. I'll guide you on how.

Let's begin by clicking the Gear icon at the top right of your QuickBooks Online. Then, proceed to the following instructions below:

1. Choose Custom fields under LISTS.
2. Hit on the Add field button.
3. Enter the name, then hit the All Sales forms box.
4. Tap Save.

Moreover, you can also utilize the Description field 

of the invoice to enter the P/O number for a particular customer.

You'll want to run a Purchase Order report to track your purchases.

The Community space has the resources you need for any invoice-related inquiries. Should you have further concerns, I'm available to assist you 24/7. Leave a comment below. Take care.