Adding line items to transactions

I want to add one line item to an opportunity once the user has selected the customer. I have the following code:

function fieldChanged(type, name)
{
if(name == 'entity')
{
  nlapiSelectNewLineItem('item');
  nlapiSetCurrentLineItemValue('item','item','xxxx',false,true);
}
return true;
}

where xxxx is the internalid of the item.

I can see the line item added. But the item field alone does not get populated. The other fields are retrieved from the item definition and get populated correctly.

Also I read other articles regarding the nlapiCommitLineItem. If i set the synchronous to true the adding of the line item happens and then vanishes!