It’s complicated. For your own security, we do not retain credit card payment information in our system and as such have no way to charge you for the additional item(s).
You will need to contact Customer Support [link] to cancel your original order, then place a new order for all desired items.
If an order you plan to cancel contains limited edition or out of stock items or coupon codes you will lose the ability to re-order those limited items or use your coupon code on your new order.