Sql-server – SUM (CASE WHEN) AS.. query

You could use a subquery but you don't need to. Just don't sum the bonus and add it in the GROUP BY list.
Notice that you have to also add the student.id, even in your original query, in case you have 2 students with same name.
You probably also need coalesce() for students without any scores:

SELECT st.name, 
       coalesce(sum(sc.score1),0) + coalesce(sum(sc.score2),0) + st.bonus AS total
FROM student st
LEFT JOIN score sc ON sc.student_id = st.id
GROUP BY st.id, st.name, st.bonus ;

In newer versions of Postgres, you could use only the primary key of the student table in the group by:

SELECT st.name, 
       coalesce(sum(sc.score1),0) + coalesce(sum(sc.score2),0) + st.bonus AS total
FROM student st
LEFT JOIN score sc ON sc.student_id = st.id
GROUP BY st.id ;

If you want a subquery, this is one way:

SELECT st.name, 
       coalesce(sc.score, 0) + st.bonus AS total
FROM student st
LEFT JOIN 
    ( SELECT student_id, sum(score1) + sum(score2) AS score
      FROM score 
      GROUP BY student_id
    ) AS sc ON sc.student_id = st.id ;

I think, in your case a derived table is necessary to isolate querying number of mutations of postcodes per customer:

SELECT c.postalcode 
, sum(s.SaleAmount) SaleAmount
, count(postcode_mutations.customerid) as CntCustomerChangedPostCode   
FROM dbo.Sale s
JOIN dbo.Customer c on s.customerid = c.customerid

LEFT JOIN (
SELECT 
    CustomerID
FROM [dbo].[Customer]
FOR SYSTEM_TIME FROM '20140101' TO '20150101'
GROUP BY CustomerID
HAVING COUNT(DISTINCT PostalCode) > 1
) postcode_mutations on s.customerid = postcode_mutations.customerid

WHERE s.SaleDateTime >= '2014-1-1' AND s.SaleDateTime < '2015-1-1'
GROUP BY c.PostalCode

upd: Since the query is supposed to serve DWH/Analytics scenarios, the columnstore indexing is an option to check. I also made some benchmarks previously for a 10 million rows table.