How can I modify an invoice template to include the customer's purchase order number in the top section of the form?

Hello Accounts38;

You will need to turn on "Custom Fields" within your account and settings (Sales Section), and then enter the purchase order number into one of the custom fields.

This is not an automatic process; you'll need to enter the PO number manually.

When you've turned this on, you'll be able to include it within your Custom Form Style by clicking on Cog-Wheel - Custom Form Styles - then Edit on the template you want to modify.