$(document).ready(function() {
   
   // disable the courier field unless the user
   // chooses "yours"   
   if (!$('input[name=shipment][value=yours]').attr("checked"))
      $('#id_courier').attr("disabled", "disabled")
   
   $('input[name=shipment]').change(function() {
      
      if ($(this).val() == "yours") {
         
         $('#id_courier').removeAttr("disabled");
         $('#id_courier').focus()
         
      } else {
         
         $('#id_courier').attr("disabled", "disabled");
      }
   });
});