inapp purchase in android

public class MainActivity extends Activity implements OnClickListener {
	Button btn1;
	private Context mContext=this;
	private static final String TAG = "Android BillingService";
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		btn1 = (Button) findViewById(R.id.button1);
		
		btn1.setOnClickListener(this);
		
		
		 startService(new Intent(mContext, BillingService.class));
	        BillingHelper.setCompletedHandler(mTransactionHandler);
	}
	public Handler mTransactionHandler = new Handler(){
		public void handleMessage(android.os.Message msg) {
			Log.i(TAG, "Transaction complete");
			Log.i(TAG, "Transaction status: "+BillingHelper.latestPurchase.purchaseState);
			Log.i(TAG, "Item purchased is: "+BillingHelper.latestPurchase.productId);
			
			if(BillingHelper.latestPurchase.isPurchased()){
				showItem();
			}
		};
	
};
	@Override
	public void onClick(View v) {
		if (v == btn1) {
			if(BillingHelper.isBillingSupported()){
				BillingHelper.requestPurchase(mContext, "android.test.purchased"); 
					        }
			else {
	        	Log.i(TAG,"Can't purchase on this device");
	        	btn1.setEnabled(false); // XXX press button before service started will disable when it shouldnt
	        }
			Toast.makeText(this, "Purchase Books", Toast.LENGTH_SHORT).show();
		}
		
	}

	private void showItem() {
		
		
		//purchaseableItem.setVisibility(View.VISIBLE);
	}

	@Override
	protected void onPause() {
		Log.i(TAG, "onPause())");
		super.onPause();
	}
	
	@Override
	protected void onDestroy() {
		BillingHelper.stopService();
		super.onDestroy();
	}
}

<uses-sdk
        android:minSdkVersion="8"
        android:targetSdkVersion="19" />
    <uses-permission android:name="android.permission.INTERNET"/>
    <uses-permission android:name="android.permission.BILLING"/>
    <uses-permission android:name="com.android.vending.BILLING" />
    <uses-permission android:name="android.permission.READ_PHONE_STATE"/>
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"></uses-permission>