Woocommerce checkout page – custom field checkbox value into email

I don’t know how I missed the answer, it’s way more simple than the stuff I was trying.

From the docs:

/* To use: 
1. Add this snippet to your theme's functions.php file
2. Change the meta key names in the snippet
3. Create a custom field in the order post - e.g. key = "Tracking Code" value = abcdefg
4. When next updating the status, or during any other event which emails the user, they will see this field in their email
*/
add_filter('woocommerce_email_order_meta_keys', 'my_custom_order_meta_keys');

function my_custom_order_meta_keys( $keys ) {
     $keys[] = 'Tracking Code'; // This will look for a custom field called 'Tracking Code' and add it to emails
     return $keys;
}

in my case

add_filter('woocommerce_email_order_meta_keys', 'client_already_email_yes');

function client_already_email_yes( $keys ) {
     $keys[] = 'Client Already:YES'; // This will look for a custom field called 'Tracking Code' and add it to emails
     return $keys;
}

This returns a value of 1 from the checked checkbox and sticks it at the end of the text in the email. It’s not pretty but good enough.