If you would like to add some items after complete order, and expect items to be delivered together, you can just make a new order with the items you want to add on. Then, follow below steps:
1.  Write a note at cart page with these information - last order number, proper instruction like "Deliver together with order xxxx"
2.  Choose "Self-Collection" at check out to remove the extra shipping charge
3.  Choose the same payment method as last order
4.  Complete order by cut-off time on the same order day
We will consolidate the orders and arrange the delivery.