контроллер
if ((float)$result['special']) {
$special_discount_diff = round($this->currency->format($this->tax->calculate($result['price'], $result['tax_class_id'], $this->config->get('config_tax')), $this->session->data['currency']) - $this->currency->format($this->tax->calculate($result['special'], $result['tax_class_id'], $this->config->get('config_tax')), $this->session->data['currency']));
} else {
$special_discount_diff = false;
}
// . . .
// ниже
// в массиве товаров
$data['products'][] = array(
'special_discount_diff' => $special_discount_diff, // вот эту строчку добавляем
шаблон
{% if product.special_discount_diff %}
{{ product.special_discount_diff }}
{% endif %}