Currently the feature to add a coupon to an order from the order_ui admin area is broken.
The form provides an 'amount' and 'currency' (based on the shipping module?) that doesn't actually work.
This feature is marked as "TODO" in the code.
The attached patch fixes that issue and provides the ability to add commerce coupon line items from the admin order edit form.
Please review and commit when possible.
P.S Thanks for the useful module!