WooCommerce classic transactional emails

Choose the smallest extension point:

1. Change markup only: override a template in the theme.
2. Change existing email values: use settings or a targeted filter.
3. Add a new notification type: extend WC_Email and register it.

Template override

your-theme/woocommerce/emails/customer-processing-order.php
your-theme/woocommerce/emails/plain/customer-processing-order.php

Copy from WooCommerce's templates/emails tree and preserve the template version header. Theme overrides win over plugin/core templates and must be reviewed when WooCommerce reports them outdated.

For a plugin-owned custom email, set: