Is it possible to disable/enable a jquery function on change of radio button

I am working on the checkout part of an online store. I created a script to be triggered once a form was submitted. i want this script to activate only if the selected payment method is not paypal since i want to skip the ajax process.

I am using codeigniter framework

Here's the code for the view

<form  action="<?php echo site_url('products/order')?>" method="post" id="checkoutForm" name="checkoutForm">
   ...
   <input type="radio" name="paymentMethod" value="cod">CASH ON DELIVERY
   <input type="radio" name="paymentMethod" value="paypal">PAYPAL
   ...
   <input type="submit" id="btnOrder" value="ORDER NOW">
</form>

<script type="text/javascript">
  var paymentMethod = '';
  $(document).ready(function(){
      $('input:radio[name="paymentMethod"]').change(function(){
           paymentMethod = $(this).val();

      });

   if(paymentMethod != 'paypal' && paymentMethod != ''){
      $('#checkoutForm').submit(function(){
         ...
         $.post($('#checkoutForm').attr('action'), $('#checkoutForm').serialize(), function( data ) {
           ...  
         }, 'json');
      return false;     
      });
    }
  });
</script>

Here's the code for the function in my controller (products.php)

public function order() {
    $paymentMethod = $this->input->post('paymentMethod');
    ...
    if($paymentMethod != 'paypal'){
        echo json_encode(array('stat' => 1,'msg' => 'Registered Successfully', 'order_id' => $order_id));

     }else{
        // ... Some paypal configurations
        $this->paypal->pay(); //Proccess the payment
    }


} # End order

What i've tried so far was to put an if statement outside the $('#checkoutForm').submit to check if radio button value is not paypal, but it's not working.

Thanks in advance