@RequestMapping(value = "/checkout/billing", method = RequestMethod.GET)
    public String getBillingPage(Model model, final HttpServletRequest request) throws CMSItemNotFoundException {
   // other code

   checkoutForm.setCustomFieldsForm(customFieldsForm);
   model.addAttribute("checkoutForm", checkoutForm);

   // other code
}

@RequestMapping(value = "/checkout/billing", method = RequestMethod.POST)
    public String submitPayment(
            @Valid @ModelAttribute("checkoutForm") final CheckoutForm checkoutForm,
            final BindingResult bindingResult,
            Model model,
            final HttpServletRequest request,
            final HttpServletResponse response) throws CMSItemNotFoundException
    {}