Firstly; it was a sad day when I realised that some letterboxes hate geocaches and anything to do with them.
Secondly; some people moan about everything and you should not go out of your way to please everyone.
One cache log I read recently, whilst sitting in evening sun miles from anywhere said “Found whilst letterboxing. What a lovely idea, we’ll join up when we get home to Cornwall”
Thirdly; Many letterboxers would not want the coordinates of their boxes posted publically. I suspect you already know this and are testing the water… 🙂