Help With Select / Sum / Nulls

i have the following select statement, the balance due column allows nulls and I am trying to eliminate all rows where total or credit is null. Is the only way to run the two select statements again in the where clause?

select
c.CustomerID ConsultantID,
c.CustomerFullName CustomerFullName,
sum(i.BalanceDue) from Invoice i where i.CustomerID = c.CustomerID) as Total,
sum(i.BalanceDue) from Invoice i where i.CustomerID = c.CustomerID and TransactionTypeID = 'Credit Memo') as Credit

from
Customer c

order by ConsultantID

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The following gives me what I want but I am sure there must be a better way, any pointers would be appreciated:)

select
c.CustomerID ConsultantID,
c.CustomerFullName CustomerFullName,
ISNULL((select sum(i.BalanceDue) from Invoice i where i.CustomerID = c.CustomerID), 0) - as Total,
ISNULL((select sum(i.BalanceDue) from Invoice i where i.CustomerID = c.CustomerID and TransactionTypeID = 'Credit Memo'), 0) as Credit

from
Customer c
where (ISNULL((select sum(i.BalanceDue) from Invoice i where i.CustomerID = c.CustomerID), 0) <> 0
or
ISNULL((select sum(i.BalanceDue) from Invoice i where i.CustomerID = c.CustomerID and TransactionTypeID = 'Credit Memo'), 0) <> 0)

order by ConsultantID
GO