Glad you like it. I think it would be a killer feature which I also don’t believe other apps have.
A push notification would be best, but as usual the ability to enable/disable it if you want.
An alert or section in the app itself also makes sense in case you miss the push.
To start with I would have the reminder set to 14 days as default, but allow the user to change this as they wish.
If the 14 day (or whatever you set) reminder triggers. It should trigger again every X many days until the refund is logged in Emma or the user removes the refund reminder from the original transaction.