Hi,
The response URL is generated in the servired plugin code by the lines:
$url_OK = HIKASHOP_LIVE . "index.php?option=com_hikashop&ctrl=checkout&task=after_end&order_id=" . $order->order_id.'&lang='.$this->locale;
$url_KO = HIKASHOP_LIVE . "index.php?option=com_hikashop&ctrl=order&task=cancel_order&order_id=" . $order->order_id.'&lang='.$this->locale;
$urlMerchant = HIKASHOP_LIVE . 'index.php?option=com_hikashop&ctrl=checkout&task=notify¬if_payment='.$this->name.'&tmpl=component&lang='.$this->locale . $this->url_itemid;
And the HIKASHOP_LIVE constant comes from the "live_site" parameter of your joomla configuration.php file. So edit it, and make sure that the URL you have there is the one you want for the main URL of your website. You'll then get that URL with the protocol you used in the live_site parameter for the response URLs.