Tell us what you are trying to do?
I want to change woocommerce orders by adding fields to customers and invoices. For example id number and vat number.
Is there any documentation that you are following?
Can't find.
I'm assuming you are referring to the order complete screen where you get the invoice details .
Unfortunately this page you would need to modify it using the woocommerce hooks . Take a look at my screenshots and could you let me know which one of these pages you are referring to exactly.
One is the cart page and the other is the invoice after you've placed an order.