Is is possible to combine local variables in a case statement?
Is there any reason you aren't just using a group by?

SELECT T0.[Project], T3.[Name] AS [Season],
     sum(T0.[LineTotal]) AS [Season Total]
FROM RDR1 T0  
INNER JOIN ORDR T1 ON T0.[DocEntry] = T1.[DocEntry]
INNER JOIN [@SES_SSNSBRG] T2 ON T0.[ItemCode] = T2.[U_SES_ItemCode]
INNER JOIN [@SES_SEASONS] T3 ON T2.[U_SES_SEASON] = T3.[Code]
GROUP BY T0.[Project], T3.[Season];