You can add any custom field to your order emails by hooking in and specifying the name of the custom field. This can be useful when used in conjunction with the Checkout Field Editor extension, or if you want to include things like the ‘transaction key’ from PayPal orders, for example.
Here is an example using the Checkout Field Editor extension of the WooCommerce > Checkout Field settings for a custom field created labeled ‘hear_about_us’:
Using this example the code would be:
$fields['How did you hear about us?'] = 'hear_about_us';