@Skyboy
Let me stand by your side. I’m also developing totally offline (scientific and educational) apps with UE.
To be more precise I work in UE 4.25. Now, at September 2024, GooglePlay forces all developers to update all apps with targetSDK 33+, which requires full recompile of apps, so all these problems become mine too. Yes, despite I don’t use this lib, I tried to just update this IAP Billing lib, like shown in this popular answer, but it’s now use for UE 4, because it’s bound to Android NDK b21e, and Gradle 3.5.4 with corresponding old build tools, not compatible with new billingclient 6.1.0 (required by GP Console at this moment).
So, if you, like me, developing offline apps, without any monetization and ads, and want just to DISABLE IAP in jour project, in UE 4 and UE 5, add this lines in your AndroidEngine.ini (Reference) and recompile (pack) your project.
[Project_Dir]\Config\Android\AndroidEngine.ini
[OnlineSubsystem]
DefaultPlatformService=GooglePlay
[OnlineSubsystemGooglePlay.Store]
bSupportsInAppPurchasing=False