When a customer completes checkout on a Drupal Commerce website, the cart order is "placed." No additional changes can be made to the order by the customer. Administrative users, with access permissions, can typically edit these "placed" orders. However, significant changes like the addition/removal or an order item or an order item quantity/pricing change can be problematic. The changes can be made, but... afterwards, things like taxes, discounts, shipping charges, etc. may not be correct.