<option value="price:asc" {if $orderby eq 'price' AND $orderway eq 'asc'}selected="selected"{/if}>{l s='Цена: Сначала дешевые'}</option>

<option value="price:desc" {if $orderby eq 'price' AND $orderway eq 'desc'}selected="selected"{/if}>{l s='Цена: Сначала дорогие'}</option>

$this->cat_products = $this->category->getProducts($this->context->language->id, (int)$this->p, (int)$this->n, $this->orderBy, $this->orderWay);

if (Tools::getValue('orderby') === 'reduction')
{
    foreach ($this->cat_products as &$row)
        $row['price_tmp'] = $row['price_without_reduction'] - $row['price'];
    
    if (Tools::getValue('orderway') === 'asc')
        uasort($this->cat_products, 'cmpPriceAsc');
    else
        uasort($this->cat_products, 'cmpPriceDesc');
}

http://ваш-магазин.ru/3-название-категории?orderby=reduction&orderway=desc

$this->addColorsToProductList($products);

if (Tools::getValue('orderby') === 'reduction')
{
    foreach ($products as &$row)
        $row['price_tmp'] = $row['price_without_reduction'] - $row['price'];
    
    if (Tools::getValue('orderway') === 'asc')
        uasort($products, 'cmpPriceAsc');
    else
        uasort($products, 'cmpPriceDesc');
}