It finally worked…!!!
After going around in circles for a few more days of tweaking different options, and even trying a popular game’s IAP gave the same purchase failure, so clearly it was not just the specific app. No idea if any of these steps fixed it but I was paying from an existing balance not with a card, the existing balance was even shown and I could select it, the payment error only came up AFTER I clicked to do the purchase.
I did find for others that some in-app purchases work and some cause problems, issues on GitHub · Where software is built - I tried to install newer MicroG Services and MicroG Companion via F-droid yesterday using the instructions but both failed as incompatible - hardly surprising since I use /e/OS official which has microG bundled with it.
[At least part of the problem with purchases was caused by me needing to switch google accounts to use a balance I already had (redeemed gift voucher on the wrong account!), and setting this account’s country correctly (UK), which you can only do if your phone location is detected by Google as being in that country. I have microG set to allow location access but after it worked nothing showed as recently using it. MicroG Companion I gave Location access to only when using the app, and MicroG Services all the time. I could only get microG using the right google account after removing the other and clearing the cache, and restarting. In MicroG settings, Location, I set wifi, hot spot, online service, nominatim on and everything else including use /e/OS’s location system to off. Advanced Privacy, Geolocation can be set to a specific location for certain apps which might help set the correct country in play.google.com and there’s also pay.google.com for Payments. This then showed the right account when I tried to do in-app payments but it still didn’t work!]
I’m not sure I did anything specific to fix it, although I did open MicroG from settings and in Google device registration I set Profile to Real (no idea if device registration is needed). In Play store services on I set everything to On.
On play.google.com website when signed in on the account I was paying with, I also picked to Require verification for
Only app installs through play.google.com
In my phone’s Settings, on my default app store (Aurora Store) I turned Open by default to In App rather than browser.
It worked seconds after a MicroG Companion crash alert, which I presume had cleared the cache on restarting it. I’m sure I had cleared cache on MicroG Services and MicroG Companion previously, and done a restart too. So I was super surprised when the payment suddenly worked - and when the app showed it had unlocked the paid for feature. I had Geolocation set to Random plausible at the time rather than Real location.
For debugging further,
I turned on Developer Options to get more info (in Settings, tap build number in About 7 times to see the options). Then turned on show crash dialogue which is when I got a better error message. I already had push notifications on for MicroG, and in MicroG settings, Cloud Messaging showed MicroG as on).
This then gave these errors when it waa falling when I hit purchase
Purchase failing with code 6
Google returned response code 3
Contact author on…
Looking that it seemed to suggest a location/country problem for spending my google play balance had to be in the same country the gift card was redeemed in, although the balance was in GBP because it was UK.