FOLLOWUP: By enabling SKDebugLogging and looking at console output coming from my Tablet using the Console app on my computer, i was able to see that the Unity code was actually sending the wrong identifier for the IAP- it was using the Android-specific one instead of the Apple one. Not sure why, but by using prepro directives to basically manually tell Unity which Identifier to use, i was able to get it working for App Store. Now I just need to rebuild and retest for Android to make sure that still works. Just posting this here in case anyone else hits this same problem. Because there are a TON of threads out there about people having issues with Apple IAP but nobody reported this as the reason.