How can I add a PO box to my invoice if I have the basic online QuickBooks?

You’ll have to add a custom field for the purchase order box, medsourcebilling.

This option is available on the Essentials, Plus, and Advanced versions of QuickBooks. If you’re using Simple Start, you may want to upgrade the subscription. If you prefer not to, you can use the Description column to enter the PO.

If you're using the higher version, you can follow these steps on how to add a custom field:

1. Click the Gear icon, then select Account and Settings.
2. Go the Sales tab.
3. Click the pencil (edit) icon on the Sales form content section.
4. Enter PO in the Name field under Custom fields.
5. Tick (✓) the Internal column.
6. Click Save.
7. Click Done.

Here are screenshots as your visual guide:

Here's how it looks like in the invoice form:

I'll get back to you as soon as possible if you need more help.