final update. Another Apple senior adviser called and directed me to a roundabout way to download popclip from my own log in to App Store. It seems that Mac App Store works differently from that on IOS.
However, there is still a corruption in my wife’s account that that the payment option is shown differently from web log in to that on the Mac (app store) itself. This has nothing to do with popclip but may the root cause of my wife not able to get the app for free. Apple still have not resolved this, pending further investigation.