Sorry for not getting back to you earlier.
I haven't removed Royal Mail from the app, the "non-native" integration should still work fine.
Unfortunately, to get OBA access you need to actually ship stuff using Royal Mail. As I live in Germany, that's not an option for me.
Last time I checked, Royal Mail didn't work on the "large competitor" you mentioned. Feel free to shoot me an email to firstname.lastname@example.org if you find an app that works and include the tracking ID.