When customers buy a product on your site, they often add the product to cart and checkout as usual. If they want to add related items with the main product, they will tick the boxes before hitting “add to cart” button. The proceess takes time and quite unconvinient as you customers won't instantly know how much the total is and decide if they can afford their purchase. Besides, the default procedure only allows them to update products or clear cart on the checkout page without knowing discounts for a group.