Once you activate your licence on an installation, you will then have access to print a PDF version of your invoice.
Navigate to your WordPress admin area.
Go to the plugin settings page. In this example: BookPress => Account Settings = Account
Locate the Payments section and click the Invoice button.
Alternatively, you can access the Members Dashboard here.
You will receive your login credentials after your first purchase, and for security reasons, will have to update your auto-generated password upon your first login.