Your order is received and authorized against your credit card immediately. It reaches the distributor overnight and is entered into their automated system upon receipt.
Due to the volume of orders received, it is not possible to interrupt the process to make a change to your order at this point. It has already been authorized against your card and may have begun the packing process and/or may have shipped.
You should place a new order with the additional items. Please note you may be charged fees for the additional, smaller order.