The abilty to edit an order for a customer for a number of reason has become a very important NEED. If one item is out of stock or the customer placed an order for the wrong item our only current option is to cancel the item and have the customer order a different item or we do it for them. Most mainstream eCommerce Site Management systems allow an order to be edited after the order has been submitted to include removing items, altering items and adding items.
Making additional payments and applying Credits can be done in the system or outside but the ability to alter the order allows for good customer service and customer experience.
Adding to this... when updating the order, the tax needs to recalculate, or at least a button to recalculate the tax & shipping as an option.