You may try this, But would have to check the performance difference between two as data is with you.
select
sum(Case when M.tax= 0 then M.amount else 0 end) As var1
,sum(Case when M.tax= 4 then M.amount else 0 end) As var2
,sum(Case when M.tax= 13.5 then M.amount else 0 end) As var3
from t_bill_item M where M.tax in (0,4,13.5);