Singleton class that fixes the issue with the android-inapp-billing-v3 library. This fix allows you to flawlessly use the BillingProcessor in multiple activities, without encountering this issue: https://github.com/anjlab/android-inapp-billing-v3/issues/503

BillingHolder.java

package com.your.package.name;

import android.app.Activity;

import com.anjlab.android.iab.v3.BillingProcessor;


// This singleton class is designed to fix this issue: https://github.com/anjlab/android-inapp-billing-v3/issues/503
// the solution is based on this suggestion: https://github.com/anjlab/android-inapp-billing-v3/issues/501#issuecomment-962612355
public class BillingHolder {
    private static BillingHolder single_instance = null;

    private BillingProcessor bp;
    public String lastBillingActivity = "";

    private BillingHolder()
    {
    }

    public static BillingHolder getInstance()
    {
        if (single_instance == null)
            single_instance = new BillingHolder();

        return single_instance;
    }

    public void killLast(){
        if (bp != null) {
            bp.release();
            bp = null;

        }
    }

    public BillingProcessor getBp(Activity activity){
        String currentActivityName = activity.getClass().getSimpleName();
        if(!currentActivityName.equals(lastBillingActivity)){
            killLast(); //to fix https://github.com/anjlab/android-inapp-billing-v3/issues/503
            return getNew(activity);
        }else{
            if(bp != null){
                return bp;
            }else{
                return getNew(activity);
            }
        }

    }

    private BillingProcessor getNew(Activity activity){
        bp = BillingProcessor.newBillingProcessor(activity, null, (BillingProcessor.IBillingHandler) activity); // doesn't bind
        bp.initialize(); // binds
        lastBillingActivity = activity.getClass().getSimpleName();
        return bp;
    }

}

Example 1:
put this in the onResume method of every activity that uses billing:

	@Override
	protected void onResume() {
		super.onResume();

		BillingHolder.getInstance().getBp(this); // must be in every activity that uses billing

	}

to use this class, simply use it directly like below.
BillingHolder.getInstance().getBp(this).purchase(this, "food");

Example 2:
put this on the onResume method of every activity that uses billing:

        public BillingProcessor bp;

	@Override
	protected void onResume() {
		super.onResume();

		bp = BillingHolder.getInstance().getBp(this); // must be in every activity that uses billing

	}

usage:
bp.purchase(this, "food");