IssueTracker

First, I can confirm the Bug is 7.x related, because version 6.0.1 is not affected.

You can reproduce it with Google Play Billing Library 7.x for older devices (Android 6,7).

How to reproduce:

• create a simple purchase button which will purchase a product and click the button on an Android 6 or 7 device.
• result: instead of showing the purchase popup, it will crash.

The error message is: classcast exception NoClassDefFoundError com.android.billingclient.api.zzce

import java.util.function.Consumer;

public final class zzce implements Consumer {
   public final void accept(Object var1) {
       if ((BillingFlowParams.ProductDetailsParams)var1 == null) {
           throw new IllegalArgumentException("ProductDetailsParams cannot be null.");
       }
    }
}

The main problem is implements Consumer and the import
import java.util.function.Consumer;
because both are API 24+ related.

I would suggest two solutions to fix the issue:
1.) use androidx.core.util.Consumer; as import. This should be backwards compatible. 2.) or set minSDK to 24+ for BillingApi 7.x

Work-around for affected users:
inside your local build.gradle (app level) add the following lines

   compileOptions {
       coreLibraryDesugaringEnabled true // support java8 features on older devices
   }
  
dependencies {

   coreLibraryDesugaring 'com.android.tools:desugar_jdk_libs:2.1.2'
   }