Add products to the cart programmatically

-- url of the page with the problem -- : www.thankgodi.com
-- HikaShop version -- : 2.5.0
-- Joomla version -- : 3.4.4
-- PHP version -- : 5.5.9
-- Browser(s) name and version -- : Firefox 37.0.2
-- Error-message(debug-mod must be tuned on) -- : No error message

Hi guys,

I'm building a solution for my client, where I provide an interface that allows call center operators to create a cart with products on behalf of the customer (the caller). When the operator hits save, the cart is created (with products), and a mail is sent out to the customer, who when they click it, logs them in automatically, loads up their cart, and takes them straight to checkout.

In theory, this works well. In practice, I can't find out how to programmatically add items to a cart. If I try AJAX calls to something like:

"option=com_hikashop&ctrl=product&task=updatecart&quantity=1&checkout=1&product_id=X"

Then I'm just told by Hikashop that the product is not available (it is...)

What is the best way to add items to a cart without using Hikashop's various interfaces?