How do I show PO number on invoices?

You can include the purchase order number (PO#) in the Projects screen and it will display automatically on the related invoices at the top. You can also view the purchase order number directly on the Invoice Overview tab of an invoice. You can also add the PO number on Memo 1 of the invoice (Memo 2 is disabled for some invoice templates). Another option is to get a custom invoice template. Please check the details about sending a custom report/invoice request.