Pre-populate Commerce Shipping Cart using a link
You could use Rules for triggering an event on your link with Rules link event and maybe combine it with Rules Link to attach the link to your commerce order entities.

If you prefer to code, you'd need either to create a new menu using hook_menu or alter an existing one with hook_menu_alter and then use commerce_cart_product_add or commerce_cart_product_add_by_id to add the product to the cart user, you can retrieve the current cart from the user with commerce_cart_order_load