PO number is not showing on the customers' invoice

To enable the PO number on the customers' invoice.

Open the customers' account, on the service info screen, click the blue "Edit" button.

The "Edit Service Info" page should pop up, find the PO number box and type the number. Then save changes.

Then make sure your invoice has the PO number portion turned on. Go to settings > configuration > invoice settings and make sure PO is chosen. Then it should go out on the invoices that have PO numbers.