Example

$articule = new ArticleEntity($post_data['articule_id']);
$product_type = Converter::classWithNamespaceToUnqualifiedShort($articule);

$current_amount = 0;

$current_item_in_cart = ModuleCart::getCurrentCartItems($product_type);
if ($current_item_in_cart) {
    $current_amount = $current_item_in_cart->getAmount();
}

$cart_item = new CartItemEntity();
$cart_item->setAmount($current_amount + $post_data['amount']);
$cart_item->setItemId($articule->getId());
$cart_item->setItemType($product_type);

ModuleCart::addItem($cart_item);