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add_filter('woocommerce_order_item_name', 'add_discount_info_to_order_emails', 10, 3);

function add_discount_info_to_order_emails($item_name, $item, $is_visible) {
    $order = $item->get_order();
    $total_discount = 0;
    $order_total = 0;

    // Calculate the total discount from order fees (adjust according to how discounts are stored in your orders)
    foreach ($order->get_items('fee') as $fee) {
        if ($fee->get_total() < 0) { // Assuming discounts are negative.
            $total_discount += abs($fee->get_total());
        }
    }

    // Calculate the total order value excluding shipping, taxes, and discounts.
    foreach ($order->get_items() as $line_item) {
        $order_total += $line_item->get_total(); // Get total excluding taxes.
    }

    // Avoid division by zero
    if ($order_total <= 0 || $total_discount <= 0) {
        return $item_name;
    }

    // Calculate this item's share of the discount.
    $item_total = $item->get_total();
    $item_discount = ($item_total / $order_total) * $total_discount;

    // If there's a discount, format it and append it to the item name.
    if ($item_discount > 0) {
        $discount_formatted = wc_price($item_discount);
        $item_name .= sprintf('<br><small>%s discount applied.</small>', $discount_formatted);
    }

    return $item_name;
}