Unfortunately, it is not possible to add extra products to an order that has already been placed. Once an order is completed, it is immediately processed, meaning changes can no longer be made afterwards.
If your order has not yet been shipped, customer service may be able to cancel the entire order for you. Once you have received confirmation of this, the amount paid will be refunded as soon as possible. You can then place a new order with all the desired products.
Would you prefer not to wait for cancellation? In that case, you can also choose to place an additional order for the missing items. Please note that these will be processed and delivered separately.