SELECT
    invoiceheader.id,
    invoiceheader.customerlastname,
    invoiceheader.customerfirstname,
    invoiceheader.customernit,
    invoicedetail.productid,
    invoicedetail.quantity,
    invoicedetail.saleprice,
    (quantity * saleprice) as Total
FROM invoiceheader
INNER JOIN invoicedetail
    ON invoiceheader.id = invoicedetail.invoiceid

InvoiceHeader::select('invoiceheader.id', 'invoiceheader.customerlastname',
   'invoiceheader.customerfirstname', 'invoiceheader.customernit',
   'invoicedetail.productid', 'invoicedetail.quantity',
   'invoicedetail.saleprice', DB::raw('(quantity * saleprice) as Total'))
->join('invoicedetail', 'invoiceheader.id', '=', 'invoicedetail.invoiceid')
->get();