I need this too.
I have a 10% discount code and some people forget to click it, then ask for it to be added after paying. Right now I have to cancel their order and then get them to redo it with the discount, it would be really helpful to be able to edit their order for them.
Shopify...WAKE UP! Why is there endless threads requesting basic functions, for over a year and in many cases several years that have never been implemented to the system. It is absolutely ridiculous that we cannot even do something as simple as discounting an order after it has been placed.
We The People deserve better than this!
I'm so sick and tired of constantly googling how to do something so basically simple only to find that it is not possible unless you pay for some third-party app, and 10 pages of threads of people all requesting the same thing. This is not new, as it has continued to occur for several years. It is beyond unacceptable.
Please do your job and implement these basic functions that we all consistently need so that we can run our businesses.