Adding a product to a contact via API?

Adding a product to a contact via API?

Hi,
My customers purchase products through my website. I want to use the Zoho CRM API to update a contact's product list and add the new product, once the product is purchased. The products and contacts are already defined in Zoho, I just need to create a new record in the table linking customers to products (double lookup insert, but into which object? how?)

Is there an API call for that? An example would be a great help.

Thanks,
Barak