Saved orders will stay in your account for 14 days. These orders can be found on the tab next to your order history in My Account.
Unfortunately, you cannot add items to saved orders. If you wish to send more items in your order, you will need to scan all items back again and include the new items you wish to sell.
Alternatively, you can send them in 2 separate orders but the orders will need to be packaged and shipped separately.