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1 hour ago, Jurist said:

Thanks for you reply. Obviously I cleared cache. I read that overriding some classes is not allwed (like Customer.php or Product.php) for example. That's why I asked my question, since I would like to know, how to implement changes, like combination price impact not being discounted, so it's not removed when one click update is done. 

I've used the Product.php as overrided class in PS 1.7

<?php
class Product extends ProductCore {
	public static function newPriceCalculation($id_shop, $id_product, $id_product_attribute, $id_country, $id_state, $zipcode, $id_currency,
        $id_group, $quantity, $use_tax, $decimals, $only_reduc, $use_reduc, $with_ecotax, &$specific_price, $use_group_reduction,
        $id_customer = 0, $use_customer_price = true, $id_cart = 0, $real_quantity = 0)
    {
        static $address = null;
        static $context = null;

        if ($address === null) {
            $address = new Address();
        }

        if ($context == null) {
            $context = Context::getContext()->cloneContext();
        }

        if ($id_shop !== null && $context->shop->id != (int)$id_shop) {
            $context->shop = new Shop((int)$id_shop);
        }

        if (!$use_customer_price) {
            $id_customer = 0;
        }

        if ($id_product_attribute === null) {
            $id_product_attribute = Product::getDefaultAttribute($id_product);
        }

        $cache_id = (int)$id_product.'-'.(int)$id_shop.'-'.(int)$id_currency.'-'.(int)$id_country.'-'.$id_state.'-'.$zipcode.'-'.(int)$id_group.
            '-'.(int)$quantity.'-'.(int)$id_product_attribute.
            '-'.(int)$with_ecotax.'-'.(int)$id_customer.'-'.(int)$use_group_reduction.'-'.(int)$id_cart.'-'.(int)$real_quantity.
            '-'.($only_reduc?'1':'0').'-'.($use_reduc?'1':'0').'-'.($use_tax?'1':'0').'-'.(int)$decimals;

        // reference parameter is filled before any returns
        $specific_price = SpecificPrice::getSpecificPrice(
            (int)$id_product,
            $id_shop,
            $id_currency,
            $id_country,
            $id_group,
            $quantity,
            $id_product_attribute,
            $id_customer,
            $id_cart,
            $real_quantity
        );

        if (isset(self::$_prices[$cache_id])) {
            /* Affect reference before returning cache */
            if (isset($specific_price['price']) && $specific_price['price'] > 0) {
                $specific_price['price'] = self::$_prices[$cache_id];
            }
            return self::$_prices[$cache_id];
        }

        // fetch price & attribute price
        $cache_id_2 = $id_product.'-'.$id_shop;
        if (!isset(self::$_pricesLevel2[$cache_id_2])) {
            $sql = new DbQuery();
            $sql->select('product_shop.`price`, product_shop.`ecotax`,p.`usage_type`,p.`pulse_rate_value`');
            $sql->from('product', 'p');
            $sql->innerJoin('product_shop', 'product_shop', '(product_shop.id_product=p.id_product AND product_shop.id_shop = '.(int)$id_shop.')');
            $sql->where('p.`id_product` = '.(int)$id_product);
            if (Combination::isFeatureActive()) {
                $sql->select('IFNULL(product_attribute_shop.id_product_attribute,0) id_product_attribute, product_attribute_shop.`price` AS attribute_price, product_attribute_shop.default_on');
                $sql->leftJoin('product_attribute_shop', 'product_attribute_shop', '(product_attribute_shop.id_product = p.id_product AND product_attribute_shop.id_shop = '.(int)$id_shop.')');
            } else {
                $sql->select('0 as id_product_attribute');
            }

            $res = Db::getInstance(_PS_USE_SQL_SLAVE_)->executeS($sql);
            if (is_array($res) && count($res)) {
                foreach ($res as $row) {
					if($row['usage_type'] == 'usage_based')
                    $array_tmp = array(
                        'price' => $row['pulse_rate_value'],
                        'ecotax' => $row['ecotax'],
                        'attribute_price' => (isset($row['attribute_price']) ? $row['attribute_price'] : null)
                    );
					else
					$array_tmp = array(
                        'price' => $row['price'],
                        'ecotax' => $row['ecotax'],
                        'attribute_price' => (isset($row['attribute_price']) ? $row['attribute_price'] : null)
                    );
						
                    self::$_pricesLevel2[$cache_id_2][(int)$row['id_product_attribute']] = $array_tmp;

                    if (isset($row['default_on']) && $row['default_on'] == 1) {
                        self::$_pricesLevel2[$cache_id_2][0] = $array_tmp;
                    }
                }
            }
        }

        if (!isset(self::$_pricesLevel2[$cache_id_2][(int)$id_product_attribute])) {
            return;
        }

        $result = self::$_pricesLevel2[$cache_id_2][(int)$id_product_attribute];

        if (!$specific_price || $specific_price['price'] < 0) {
            $price = (float)$result['price'];
        } else {
            $price = (float)$specific_price['price'];
        }
        // convert only if the specific price is in the default currency (id_currency = 0)
        if (!$specific_price || !($specific_price['price'] >= 0 && $specific_price['id_currency'])) {
            $price = Tools::convertPrice($price, $id_currency);
            if (isset($specific_price['price']) && $specific_price['price'] >= 0) {
                $specific_price['price'] = $price;
            }
        }

        // Attribute price
        if (is_array($result) && (!$specific_price || !$specific_price['id_product_attribute'] || $specific_price['price'] < 0)) {
            $attribute_price = Tools::convertPrice($result['attribute_price'] !== null ? (float)$result['attribute_price'] : 0, $id_currency);
            // If you want the default combination, please use NULL value instead
            if ($id_product_attribute !== false) {
                $price += $attribute_price;
            }
        }

        // Tax
        $address->id_country = $id_country;
        $address->id_state = $id_state;
        $address->postcode = $zipcode;

        $tax_manager = TaxManagerFactory::getManager($address, Product::getIdTaxRulesGroupByIdProduct((int)$id_product, $context));
        $product_tax_calculator = $tax_manager->getTaxCalculator();

        // Add Tax
        if ($use_tax) {
            $price = $product_tax_calculator->addTaxes($price);
        }

        // Eco Tax
        if (($result['ecotax'] || isset($result['attribute_ecotax'])) && $with_ecotax) {
            $ecotax = $result['ecotax'];
            if (isset($result['attribute_ecotax']) && $result['attribute_ecotax'] > 0) {
                $ecotax = $result['attribute_ecotax'];
            }

            if ($id_currency) {
                $ecotax = Tools::convertPrice($ecotax, $id_currency);
            }
            if ($use_tax) {
                // reinit the tax manager for ecotax handling
                $tax_manager = TaxManagerFactory::getManager(
                    $address,
                    (int)Configuration::get('PS_ECOTAX_TAX_RULES_GROUP_ID')
                );
                $ecotax_tax_calculator = $tax_manager->getTaxCalculator();
                $price += $ecotax_tax_calculator->addTaxes($ecotax);
            } else {
                $price += $ecotax;
            }
        }

        // Reduction
        $specific_price_reduction = 0;
        if (($only_reduc || $use_reduc) && $specific_price) {
            if ($specific_price['reduction_type'] == 'amount') {
                $reduction_amount = $specific_price['reduction'];

                if (!$specific_price['id_currency']) {
                    $reduction_amount = Tools::convertPrice($reduction_amount, $id_currency);
                }

                $specific_price_reduction = $reduction_amount;

                // Adjust taxes if required

                if (!$use_tax && $specific_price['reduction_tax']) {
                    $specific_price_reduction = $product_tax_calculator->removeTaxes($specific_price_reduction);
                }
                if ($use_tax && !$specific_price['reduction_tax']) {
                    $specific_price_reduction = $product_tax_calculator->addTaxes($specific_price_reduction);
                }
            } else {
                $specific_price_reduction = $price * $specific_price['reduction'];
            }
        }

        if ($use_reduc) {
            $price -= $specific_price_reduction;
        }

        // Group reduction
        if ($use_group_reduction) {
            $reduction_from_category = GroupReduction::getValueForProduct($id_product, $id_group);
            if ($reduction_from_category !== false) {
                $group_reduction = $price * (float)$reduction_from_category;
            } else { // apply group reduction if there is no group reduction for this category
                $group_reduction = (($reduc = Group::getReductionByIdGroup($id_group)) != 0) ? ($price * $reduc / 100) : 0;
            }

            $price -= $group_reduction;
        }

        if ($only_reduc) {
            return Tools::ps_round($specific_price_reduction, $decimals);
        }

        $price = Tools::ps_round($price, $decimals);

        if ($price < 0) {
            $price = 0;
        }

        self::$_prices[$cache_id] = $price;
        return self::$_prices[$cache_id];
    }
}

It's working for me... its for your reference.. I'm just overriding class with a function and tried to check its impact.