Blind Copy an invoice

I want to automatically blind copy a transaction as an attachment.
I am doing this as an after submit script - and it works OK except the transation is not attached. In the code below 'record' is the id of the invoice I want to attach. I thought the mail merge would do this?

function BccEmail()
{
var record = nlapiGetRecordId();
var emailBody = nlapiMergeEmail( 14, 'invoice', record);
debugTxt = "Record is "+record;
nlapiLogExecution('DEBUG','<Before Load Script>',debugTxt);
nlapiSendEmail( -5, -5, 'Blind Copy of transaction form', emailBody , null, "peter@dfwilson.co.uk", null );
}

Any help appreciated