SimpleCart SimpleCart 2.x Manager Free Products
Via Extras > SimpleCart > Management you can set up products that are automatically given to users for free when they purchase a specific product.
The Product to add is the free product, while the Product restriction is the product that needs to be in the cart in order to get the free product. With the From amount you can ensure a minimum order total has been reached before adding the free product, and the Max Usage and Expire date limits the offer further.