public class MyActivity extends Activity {

    private static final String TAG = "MainActivity";
    private Button button;

    private static final int REQUEST_CODE_PAYMENT = 1;
    private static PayPalConfiguration paypalConfig = new PayPalConfiguration()
            .environment(PayPalConfiguration.ENVIRONMENT_SANDBOX)
            .clientId(Config.PAYPAL_CLIENT_ID);
    private List<PayPalItem> productsInCart = new ArrayList<PayPalItem>();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        button = (Button) findViewById(R.id.button);
        button.setOnClickListener(new View.OnClickListener() {

            public void onClick(View v) {
                addToCart(new Product("1", "sinny", "fredric_sinny", null, new BigDecimal(1), "1232131"));

                PayPalPayment thingsToBuy = prepareFinalCart();

                Intent intent = new Intent(MyActivity.this,
                        PaymentActivity.class);

                intent.putExtra(PayPalService.EXTRA_PAYPAL_CONFIGURATION, paypalConfig);

                intent.putExtra(PaymentActivity.EXTRA_PAYMENT, thingsToBuy);

                startActivityForResult(intent, REQUEST_CODE_PAYMENT);
            }
        });

    }

    private PayPalPayment prepareFinalCart() {

        PayPalItem[] items = new PayPalItem[productsInCart.size()];
        items = productsInCart.toArray(items);

        // Total amount
        BigDecimal subtotal = PayPalItem.getItemTotal(items);

        // If you have shipping cost, add it here
        BigDecimal shipping = new BigDecimal("0.0");

        // If you have tax, add it here
        BigDecimal tax = new BigDecimal("0.0");

        PayPalPaymentDetails paymentDetails = new PayPalPaymentDetails(
                shipping, subtotal, tax);

        BigDecimal amount = subtotal.add(shipping).add(tax);

        PayPalPayment payment = new PayPalPayment(
                amount,
                Config.DEFAULT_CURRENCY,
                "Description about transaction. This will be displayed to the user.",
                PayPalPayment.PAYMENT_INTENT_SALE);

        payment.items(items).paymentDetails(paymentDetails);

        payment.custom("This is text that will be associated with the payment that the app can use.");

        return payment;
    }

    public void addToCart(Product product) {
        PayPalItem item = new PayPalItem(product.getName(), 1,
                product.getPrice(), Config.DEFAULT_CURRENCY, product.getSku());

        productsInCart.add(item);

        Toast.makeText(getApplicationContext(),
                item.getName() + " added to cart!", Toast.LENGTH_SHORT).show();
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {

        super.onActivityResult(requestCode, resultCode, data);

        PaymentConfirmation confirm = data
                .getParcelableExtra(PaymentActivity.EXTRA_RESULT_CONFIRMATION);

        String paymentId;
        try {
            paymentId = confirm.toJSONObject().getJSONObject("response")
                    .getString("id");

            String payment_client = confirm.getPayment().toJSONObject()
                    .toString();

            Log.e(TAG, "paymentId: " + paymentId + ", payment_json: "
                    + payment_client);
        } catch (JSONException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}