Требуется создать категорию, куда будут попадать товары с скидкой.

• создаем категорию с адресом страницы «sale»
• в файле /view/ProductsView.php правим код:

// Выберем текущую категорию
if (!empty($category_url))
{
	$category = $this->categories->get_category((string)$category_url);
	if (empty($category) || (!$category->visible && empty($_SESSION['admin'])))
		return false;
	$this->design->assign('category', $category);
	
	if($category->url == 'sale') {
		$filter['discounted'] = 1;
	}
	else {
		$filter['category_id'] = $category->children;
	}
}

• В файде /api/Products.php обновляем методы get_products и count_products:

public function get_products($filter = array())
{
...
$is_featured_filter = '';
$discounted_filter = '';
$in_stock_filter = '';
...
if(isset($filter['discounted']))
	$discounted_filter = $this->db->placehold('AND (SELECT 1 FROM __variants pv WHERE pv.product_id=p.id AND pv.compare_price>0 LIMIT 1) = ?', intval($filter['discounted']));
...
$query = "SELECT 
...
WHERE
...
$discounted_filter
...
";
...
}
public function count_products($filter = array())
{
...
$is_featured_filter = '';
$discounted_filter = '';
$in_stock_filter = '';
...
if(isset($filter['discounted']))
	$discounted_filter = $this->db->placehold('AND (SELECT 1 FROM __variants pv WHERE pv.product_id=p.id AND pv.compare_price>0 LIMIT 1) = ?', intval($filter['discounted']));
...
$query = "SELECT 
...
WHERE
...
$discounted_filter
...
";
...
}