First, log in to your WP Simple Pay account, then in the Billing tab, click the Generate Invoice link next to the purchase you wish to generate an invoice for:
This will open a modal window where you can fill out all the fields you require, then click Save Billing Details & Generate Invoice.
This will open a new tab and display the invoice screen where you can Print or Download a PDF of the invoice.
Once your invoice is generated, you will be returned to the previous screen, where you can Print or Download your customized invoice.