Billing page options are set from the Billflow dashboard, or passed in through the embed snippet. It's necessary to pass some options through that are dynamic by nature, like the user's email or security hash. Passing in an option will override the option set from the dashboard. Following are the options available for each billing page.
If you are using any of the billing pages that utilize another billing page (eg. Plan Picker uses Checkout Form when a customer selects a plan), you can add configuration options to the billing page for both. So make sure to check out Checkout Form Configs if you're using another billing page.