SELECT fvendno, fcompany, POItemKey, PORelsdate, SUM(totstdcost) AS TotStdCost
FROM #mytable
GROUP BY fvendno, fcompany, GROUPING SETS( (POItemKey, PORelsdate, ID), () )
;

This only requires 1 scan, and 2 logical reads whereas the UNION approach requires 3 scans and 6 logical reads (2/4 if you exclude the Grand Total).

If you want to add a grand total, you can use the following grouping sets.
SELECT fvendno, fcompany, POItemKey, PORelsdate, SUM(totstdcost) AS TotStdCost, GROUPING(fvendno)
FROM #mytable
GROUP BY GROUPING SETS( (fvendno, fcompany, POItemKey, PORelsdate, ID), (fvendno, fcompany), () )
;

Drew

Edited:  Added the query for a grand total.