I would please like the option to create a flat fee on shipping that applies to the entire order as a whole, even if the items are fulfilled from different locations.
I saw one work around- to add inventory of every product to every location so it never appears that we are shipping from different locations, but unfortunately, that workaround will create more problems with inventory management and fulfillment clarity.
I've read another thread on this issue and it doesn't seem resolved.
I hope that Shopify can offer us this option soon, as our customer experiences (and thus my sales) are directly impacted by it.
You can contact shopify support and ask them to change this for you. It's not a setting in your admin, but I had this issue and talked to them, they said by request they can switch you to flat shipping per order.