public function BillingForm()
    {
        $form = BillingDetailsForm::create($this, 'BillingForm');

        $data = Session::get("Checkout.BillingDetailsForm.data");
        if (is_array($data)) {
            $form->loadDataFrom($data);
        } elseif($member = Member::currentUser()) {
            // Fill email, phone, etc
            $form->loadDataFrom($member);
            
            // Then fill with Address info
            if($member->DefaultAddress()) {
                $form->loadDataFrom($member->DefaultAddress());
            }
        }

        $this->extend("updateBillingForm", $form);

        return $form;
    }