Exclude Row If Calculated Column Is Zero?

I am attempting to create an SQL statement that will query a file and give me amount totals by company number/customer number. The totals have to be combined into 4 groups (1/2/3/4) for each amount total in company number/customer number combination. In effect it will look something like this:

COMPANY | CUSTOMER | SORT | AMOUNT
==================================
00001 | 11111 | 1 | $55
00001 | 11111 | 2 | $12
00001 | 11111 | 3 | $19
00001 | 11111 | 4 | $ 0
00001 | 22222 | 1 | $99
00001 | 22222 | 2 | $53
...and so on.

I HAVE THIS PART WORKING ALREADY. The problem is that I am trying to exclude the rows that have 0 (zero) in the amount column from showing up in the output. The amount is a calculated field of all the invoice for that company number/customer number combination for that sort (eg: Company 00001/Customer 11111/Sort 1 has $55 associated to it). I cannot use the calculated field in my where clause.

I will include a simplified version of my select statement so you can see how I got as far as I have and where to go so I pretty much say "WHERE SUM(SubTBL.Amount) <> 0".

----SELECT STATEMENT-----
SELECT
MainTBL.Cust#,
SUM(SubTBL.Amount) As TotAmt,
CASE
WHEN (days (currdate) - days (MainTBL.DateFLD)) <= 30 THEN '1'
WHEN (days (currdate) - days (MainTBL.DateFLD)) BETWEEN 31 AND 60 THEN '2'
WHEN (days (currdate) - days (MainTBL.DateFLD)) BETWEEN 61 AND 90 THEN '3'
WHEN (days (currdate) - days (MainTBL.DateFLD))> 90 THEN '4'

[code]....