Product.php

<?php

// Customization price
if ((int)$id_customization) {
    $customization_cost = Tools::convertPrice(Customization::getCustomizationPrice($id_customization), $id_currency);
    if ((int)$id_cart) {
        $discount_qty = 10;
        $discount = 50;
        $nb_total_customizations = (int)Db::getInstance()->getValue('
            SELECT SUM(quantity) 
            FROM `' . _DB_PREFIX_ . 'cart_product` 
            WHERE id_cart = ' . (int)$id_cart . ' AND id_customization != 0'
        );
        if ($only_reduc || $use_reduc) {
            if ($nb_total_customizations >= $discount_qty) {
                $customization_cost -= $customization_cost * ($discount / 100);
            }
        }
    }
    $price += $customization_cost;
}