Prestashop Override | Module Class [exclusive]

// Add custom logic BEFORE parent execution PrestaShopLogger::addLog("Custom: Validating order for cart #$id_cart", 1, null, 'Cart', $id_cart);

Always check the PrestaShop Developer Documentation to see if a Hook can achieve your goal. Hooks are safer and more compatible than overrides. prestashop override module class

[Module][ControllerName]ModuleFrontControllerOverride (for front controllers). 3. Critical Step: Clearing the Cache prestashop override module class