Richard has a current disdain for every oldies remake that they recorded after Now & Then -He feels that they never should've been recorded(Postman,Hush,Breaking Up,Beechwood).It's likely that the duo and the powers that be looked at the two possibilities of "Breaking Up Is Hard To Do" and were all still basking in the glow of "Please Mr. Postman" having topped the charts.
It's also likely that they were looking for something uptempo to conclude the album that was heavy on ballads.
Maybe Richard's current disdain for the track stems from that debate in that he might have been on the wrong side of the argument.
Just conjecture...
Harry
The only reason "Postman" gets included on so many compilations is only due to the fact that it was a #1 hit.Otherwise,that track would be relegated to the status of "Beechwood" and "Breaking Up".