@Test
  public void testPay() {
    GenericCreditCardService creditCardService = Mockito.mock(GenericCreditCardService.class);
    Purchase p = BillingCalculator.calculateTotalPurchase(customer, "EL-001,FU-002");

    // Mocking external service behavior
    Mockito.when(creditCardService.pay("xxxx111xxxx", p.getTotalPrice())).thenReturn(true);

    paymentService.pay(customer, p, "xxxx111xxxx", creditCardService);
  }