Add instructions to the email, based on the payment method

add_action( 'woocommerce_before_email_order', 'add_order_instruction_email', 10, 2 );
 
function add_order_instruction_email( $order, $sent_to_admin ) {
 
 if ( ! $sent_to_admin ) {
 
 if ( 'cod' == $order->payment_method ) {
 // cash on delivery method
 echo '<p><strong>Instructions:</strong> Full payment is due immediately upon delivery: <em>cash only, no exceptions</em>.</p>';
 } else {
 // other methods (ie credit card)
 echo '<p><strong>Instructions:</strong> Please look for "Madrigal Electromotive GmbH" on your next credit card statement.</p>';
 }
 }
}