I am trying to add the customer's purchase order number to the the packing slip. I have tried adding it to the Note to Customer field, but it does not show up anywhere.

Nice to see you posting here in Community, architee. Currently, adding a Purchase Order (PO) number to the packing slip is unavailable in QuickBooks Online (QBO). As a workaround, we can add the PO number to the description field of your invoice. I'll be glad to show you how.

1. Sign in to your QuickBooks Online account.
2. Go to + New, and select Invoices.
3. In the Description, you can put the P.O. number.
4. Once done, click Save and send.

Moreover, when the customer makes a payment on the invoice, you need to record the payment in QuickBooks.

Meeting your goal is our top priority, and I’m here to assist you whenever you need help managing your invoices in QBO. I look forward to providing you with the support you need.