[HttpPost]
public IActionResult Submit(ShipmentAddress form) {
    if (!validate(form)) {    → 유효성 검증
        return shippingFormError();
    }
    bool success = service.SaveShippingInfo(form);    → 저장
    if (!success) {    → 오류 처리
        reportSaveError();
        return shippingFormError();
    }
    return RedirectToAction("Index", "BillingForm");  → 성공적인 응답
}

private bool validate(ShipmentAddress form) {
    if (!ModelState.IsValid) {
        return false;
    }
    var validationResult = service.ValidateShippingForm(form);
    return validationResult == ShippingFormValidationResult.Valid;
}

private IActionResult shippingFormError() {
    Response.Cookies.Append("shipping_error", "1");
    return RedirectToAction("Index", "ShippingForm", form);
}

private void reportSaveError() {
    ModelState.AddModelError("", "Problem occurred while " +
    "saving your information, please try again");
}