Hi Ludvig!
At the moment this functionality is not available through our admin or through any apps offered in our app store. All orders are processed through the checkout pages so for customers to be able to login to their respective accounts and amend any orders already processed would demand a radical overhaul of our platform in how orders are processed and logged within our system so such a change is not on the horizon in the near future at least.
Direct correspondence with a customer to discuss any changes to an order is currently the best way to manage this, then using the functionality provided by apps such as the Editify app to update the order:
https://apps.shopify.com/order-editor
I would be happy to also point you towards our API forums where you can share your query with Shopify Experts and app developers and see if the functionality you wish for is something that could potentially be made available going forward:
https://ecommerce.shopify.com/c/shopify-apis-and-technology
Hope this helps!
Andrew
Shopify Guru
support@shopify.com
