Hello!
I’ve gone through a missing implementation in UE 5.5.4 for Google IAP of non-consumable (one time purchase item). I had to modify the Engine\Plugins to fix it. Let me know if you would like to use it.
Some details are at In-App Purchases. How to restore IAP in UE 5.4 - #3 by Rush_At_Games
Have fun,
Rush At Games.