Using case and group by in select statement not working
Look at this demo code :  

125342', 'Some Fake Name', 1343632, GETDATE(), 3, 'FG-PD-TJ-PQ-20.0K-FORN'),('INV000125342', 'Some Fake Name', 1343632, GETDATE(), 11, 'FG-PD-TJ-PQ-2.25K-FORN'),
('INV000125342', 'Some Fake Name', 1343632, GETDATE(), 2, 'FG-PD-TJ-PQ-2.25K-FORN'),('INV000125342', 'Some Fake Name', 1343632, GETDATE(), 7, 'FG-PD-TJ-PQ-2.25K-FORN'),
('INV000125342', 'Some Fake Name', 1343632, GETDATE(), 2, 'Other'),('INV000125342', 'Some Fake Name', 1343632, GETDATE(), 3, 'Other')


select SOPNUMBE, CUSTNAME, CUSTNBR, DOCDATE,
SUM(QUANTITY) "TotalOnInvoce", 
sum(Case when ITEMNBR='FG-PD-TJ-PQ-KILOM-FORN' then QUANTITY*30
when ITEMNBR='FG-PD-TJ-PQ-20.0K-FORN' then QUANTITY*20
when ITEMNBR='FG-PD-TJ-PQ-2.25K-FORN' then QUANTITY *2.5
end) as "TotalShippingWeight"
from #demo
group by SOPNUMBE, CUSTNAME, CUSTNBR, DOCDATE

drop table #demo


I hope this will help you.