You could use a custom function hooked in woocommerce_email_order_details
action hook, targeting "New Order" email notification (send to admin), where you will add those details this way:
add_action('woocommerce_email_order_details', 'new_order_custom_email_notification', 20, 4 );
function new_order_custom_email_notification( $order, $sent_to_admin, $plain_text, $email )
{
// Only for "New order" email notifications (admin)
if( $email->id != 'new_order' ) return;
$order_total = floatval($order->get_total());
$order_total_discount = $order_total * 0.2;
$order_total_discounted = $order_total - $order_total_discount;
// CSS style
$styles = '<style>
.discount-info table{width: 100%; font-family: \'Helvetica Neue\', Helvetica, Roboto, Arial, sans-serif;
color: #737373; border: 1px solid #e4e4e4; margin-bottom:8px;}
.discount-info table th, table.tracking-info td{text-align: left; border-top-width: 4px;
color: #737373; border: 1px solid #e4e4e4; padding: 12px; width:58%;}
.discount-info table td{text-align: left; border-top-width: 4px; color: #737373; border: 1px solid #e4e4e4; padding: 12px;}
</style>';
// HTML Structure
$html_output = '<h2>'.__('Reseller discount Information').'</h2>
<div class="discount-info">
<table cellspacing="0" cellpadding="6">
<tr>
<th>' . __('Order total') . '</th>
<td>' . wc_price($order_total) . '</td>
</tr>
<tr>
<th>' . __('Discount 20%') . '</th>
<td>' . wc_price($order_total_discount) . '</td>
</tr>
<tr>
<th>' . __('Order total discounted') . '</th>
<td>' . wc_price($order_total_discounted) . '</td>
</tr>
</table>
</div><br>'; // HTML (end)
// Output CSS + HTML
echo $styles . $html_output;
}
This will output something like:

Code goes in function.php file of your active child theme (or active theme).
Tested and works.
To display it before Order details you can change the hook priority from 20
to 9