public interface purchasesInterface() {
public void purchaseItem();
}
________________

AndroidLauncher:
public class AndroidLauncher extends AndroidApplication implements purchaseInterface{

@Override
    protected void onCreate (Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        AndroidApplicationConfiguration config = new AndroidApplicationConfiguration();
        initialize(new MainClass(this), config);
        mHelper = new IabHelper(this, base64EncodedPublicKey);

        mHelper.startSetup(new IabHelper.OnIabSetupFinishedListener() {
            public void onIabSetupFinished(IabResult result) {
                if (!result.isSuccess()) {
                    // IF any problem occurs, log that.
                    Log.d("IAB", "Problem setting up In-app Billing: " + result);
                }
                // IAB is success
                Log.d("IAB", "Billing Success: " + result);

                processPurchases();


            }
        });

    }

@Override
    public void purchaseItem() {
        // TODO Auto-generated method stub
        mHelper.launchPurchaseFlow(this, ITEM_1, RC_REQUEST, mPurchaseFinishedListener, "HANDLE_PAYLOADS");
    }

__________________
IOSLauncher:

public class IOSLauncher extends IOSApplication.Delegate implements purchaseInterface{

private InAppPurchaseManager iapManager;
private Map<String, SKProduct> appStoreProducts;

@Override
    protected IOSApplication createApplication() {
        IOSApplicationConfiguration config = new IOSApplicationConfiguration();
        config.orientationPortrait = true;
        config.orientationLandscape = false;
        return new IOSApplication(new MainClass(this), config);
    }

@Override
    public boolean didFinishLaunching(UIApplication application, UIApplicationLaunchOptions launchOptions) {
        super.didFinishLaunching(application, launchOptions);
        iapManager = new InAppPurchaseManager(new InAppPurchaseListener() {

            @Override
            public void productsReceived(SKProduct[] skProducts) {
                // Store the products.
                appStoreProducts = new HashMap<String, SKProduct>();

                for (int i = 0; i < skProducts.length; i++) {
                    appStoreProducts.put(skProducts[i].getProductIdentifier().toString(), skProducts[i]);
                }

                Gdx.app.debug("productsReceived", "Total products: " + appStoreProducts.size());
                // Fill your shop UI with products here.
            }

            @Override
            public void productsRequestFailed(SKRequest skRequest, NSError nsError) {

            }

            @Override
            public void transactionCompleted(SKPaymentTransaction skPaymentTransaction) {
                // Purchase successfully completed.
                // Get the product identifier and award the product to the user.
                String productId = skPaymentTransaction.getPayment().getProductIdentifier().toString();
                Gdx.app.debug("IOSApplication transactionCompleted", "Retrieved id: " + productId);
                Gdx.app.debug("IOSApplication transactionCompleted", "Wanted id: " + IOSpurchaseInterface.TOMBSTONE_2);
                if (productId.equals(IOSpurchaseInterface.ITEM_1)) {
                    item1 = true;
               }
            }

            @Override
            public void transactionFailed(SKPaymentTransaction skPaymentTransaction, NSError nsError) {

            }

            @Override
            public void transactionRestored(SKPaymentTransaction skPaymentTransaction) {
                // Purchase successfully restored.
                // Get the product identifier and award the product to the user. This is only useful for non-consumable products.
                String productId = skPaymentTransaction.getPayment().getProductIdentifier().toString();
                Gdx.app.debug("IOSApplication transactionRestored", "Retrieved id: " + productId);

                if (productId.equals(purchaseInterface.ITEM_1)) {
                    item1 = true;
                }
            }

            @Override
            public void transactionRestoreFailed(NSArray<SKPaymentTransaction> nsArray, NSError nsError) {

            }
        });
        // First request the available products from the app store.

         iapManager.restoreTransactions();
        return true;
    }

    public static void main(String[] argv) {
        NSAutoreleasePool pool = new NSAutoreleasePool();
        UIApplication.main(argv, null, IOSLauncher.class);
        pool.close();

    }

@Override
    public void purchaseItem() {
        iapManager.requestProducts(IOSpurchaseInterface.ITEM_1);
        // At any time you want to purchase a product.
        if (iapManager.canMakePayments() && appStoreProducts != null) {
            Gdx.app.debug("IOSApplication", "Buying products");
            iapManager.purchaseProduct(appStoreProducts.get(IOSpurchaseInterface.ITEM_1));
        }else{
            Gdx.app.debug("IOSApplication payments?", ""+iapManager.canMakePayments());
            //Gdx.app.debug("IOSApplication products", "" + appStoreProducts.size());
        }
    }

_____________________

Purchasing
Screen Class:

public class FlowerScreen implements Screen, purchaseInterface {
purchaseInterface pInt;

 public FlowerScreen(MainClass gam) {
        game = gam;
        this.pInt = game.pi;
...
...

buyBtn1.addListener(new ChangeListener() {

            @Override
            public void changed(ChangeEvent event, Actor actor) {
                purchaseItem();
            }

        });

@Override
    public void purchaseItem() {
        pInt.purchaseItem();

    }