How to add custom field value to email

The answer is yes ??

First, you copy the file paymill/lib/tpl/pay_button.php from plugin directory into your theme subdirectory: THEME/paymill/pay_button.php

You may edit/add contents e.g. adding new form fields.

Now you are adding a new filter (e.g. within functions.php of your theme or by creating your own plugin. That filter is for editing the order confirmation mail:

add_filter('paymill_paybutton_email_text', 'your_custom_mail_function', 10, 2);

/*
$order_mail: contains a text string with order mail contents
$data: contains multi dimensional array with:
- order ID
- POST Data submitted by form
*/
function your_custom_mail_function($order_mail,$data){

// do you coding stuff here

return $order_mail; // new confirmation mail text
}

You may want to work with
str_replace
or
$order_mail = $prepend.$order_mail.$append
or just create a whole new confirmation mail without using the original $order_mail contents (just return the new content and the old one will be discarded).

Thanks for the fast and detailed response. I’ll give it a try and will come back to you with the feedback