To add the Purchase Order Number of a Client to an invoice:
1. Go to the My Calendar screen.
2. Generate the necessary invoices.
3. Once the invoices have been generated, click Invoices > Invoice List.
4. In the invoice list screen, click Edit Invoice on the particular invoice you want to add the purchase order number to.
5. In the top right corner of this screen there is a box that says 'Purchase Order' and here you can input that client's PO number.
The PO numbers do not print when using QXpress, but they are inserted for printing when the invoice is exported into Quickbook