When sales orders are created with empty shipping on CE without freight amount, they get integrated successfully to FnO without a problem. However, when customers try to edit and add freight amount, an error is produced that "shipping site is mandatory." When this was escalated to PG, they provided the explanation below:

This is by design.  

When customer create sales order line in CE and sync to F&O, the site and warehouse will automatically set in F&O (maybe from sales order header), but the site and warehouse are still blank in CE(since customer hasn't set), when a new sales order line update request send to F&O, the blank site and warehouse sent to F&O, it is not allowed to update site and warehouse to blank in F&O. 

Customer can use button "Price Order" on sales order header to sync F&O data(including site and warehouse) back to CE and then do other updates. 

The logic to set default value for warehouse and site on sales order line is different from F&O and CE. 

In CE, no default logic for warehouse and site. In F&O, it is complicated, it is based on default order setting on item, warehouse and site on sales order header. 

We don't have information about default order setting on item in CE, so we cannot add same default logic in CE like F&O. 

The suggested idea is to adjust the way integration works the same way with freight amount added.