I've been trying to insert a line item (insurance cost) after the user clicks on Save on a transaction. First, I did it with a client script, but the sync issues were too constricting. So I then moved to a User Event script (beforeSubmit). Here is the code:

function beforeSubmit() {
  addInsuranceLine();
}

function addInsuranceLine() {
  var record = nlapiGetNewRecord();

  record.insertLineItem('item', record.getLineItemCount('item'));
  record.setLineItemValue('item', 'item', record.getLineItemCount('item'), 21);
  record.setLineItemValue('item', 'quantity', record.getLineItemCount('item'), 1);
  record.setLineItemValue('item','rate', record.getLineItemCount('item'), 0.01);
  record.setLineItemValue('item','amount', record.getLineItemCount('item'), 0.01);
}

Note that 21 is the internal ID of the item I want to add.