How do I add a purchase number to Invoices?

Studiometry has a field in the Project Info panel for a Purchase Order Number. Just enter your number in here.

When you create your invoice you'll have to select a template. If the template includes the ProjectPurchaseOrderNumber tag then the information you've entered will appear. If your template does not include that variable, you can add it by modifying your invoice templates in the Preferences Window > Templates section.