HI There,
Thank you for the opportunity.
You are looking to Add additional product to the cart when ordered functionality on your existing site.
So for example, when ordering product x, product y is automatically added to the cart BUT this is totally invisible to the customer, s/he won't see it in the cart, (including the cart drop-down), checkout, transactional emails sent to customer or the in the orders in the My account section
Looking forward to discussing in detail.
Thanks
Mirth