Unfortunately, combining multiple orders placed by the same customer is not possible.
This is due to the production process. Any order placed is transfered to the lab, usually within minutes after the payment has been confirmed. This generates a lab ID which is unique to each order. If a second order is placed, this order receives another, different lab ID.
Orders are as such transferred seperately to the lab and are also produced seperately. Due to the production processes, it is not possible for labs to combine orders from the same customer.
Please note that the customer has to also pay shipping fees twice.