[HttpPost]
public IActionResult Submit4(ShipmentAddress form) {
    IActionResult error() {    → 로컬 함수
        Response.Cookies.Append("shipping_error", "1");
        return RedirectToAction("Index", "ShippingForm", form);
    }
    if (!ModelState.IsValid) {
        return error();    → 공통적인 오류 반환
    }
    var validationResult = service.ValidateShippingForm(form);
    if (validationResult != ShippingFormValidationResult.Valid) {

        return error();    }
    bool success = service.SaveShippingInfo(form);
    if (!success) {
        ModelState.AddModelError("", "Problem occurred while " +
        "saving your information, please try again");
        return error();

→ 중복된 종료 지점
    }
    return RedirectToAction("Index", "BillingForm");
}