Changing promocode through webstore.

Hi, guys!

I've got a problem with changing of promocode field when shopper is making an order.  I need the following:
There are some goods on sale (custitem_field) when shopper goes to checkout page and enters coupon code, the script scans the shopping cart and checks if there exist goods on sale. If yes, then promocode mustn't be used. Everything is good but still  somewhere there're data about promocode. Please share your ideas how to change promocode in this issue the right way.

The example of this code:

 function customFieldChanged(type,name, linenum){      if (name=='promocode'){          if (checkItems()){              nlapiSetFieldValue('promocode','',true,true);              nlapiSetFieldValue('discountitem','',true,true);              nlapiSetFieldValue('custbody_webysuite_infomessage',getInfoMessage(),true,true);              nlapiLogExecution('DEBUG','customFieldChanged','no code');          }          }  }