You may receive several orders from one customer and want to merge into one order. So when it place to Topdser it will calculate the shipping fee as one package and get a cheaper shipping fee than separated orders.
For example, you have 3 orders from one customer, order#1072, #1071 and #1070
Firstly, void order #1072, #1071 on Topdser and keep #1070.
Then go to Shopify and find the order #1070, click "edit" and add the same items as #1072 and #1071
After you add the item, click "Update Order"
You will see all 3 items are merge into order#1070. Please make sure all items are mark as paid on Shopify.
Then Topdser will sync the new added item to #1070 and you can order all items in one Shopify order.