Try editing the letter (you didn't specify the particular one) then go to Report-Filters . You should see the expression editor window come up and it will show something like the following line

AUDIT_SPLIT.AGED_CUM_30 > 0 AND SALES_LEDGER.RESTRICT_MAIL = 0  and COMMUNICATION_ADDRESS.LETTER_TYPE_ID = 1

What I think you need to do is change the CUM_30 > 0  to be  <> 0

It worked for me with a customer who had a credit balance.

Try editing the letter (you didn't specify the particular one) then go to Report-Filters . You should see the expression editor window come up and it will show something like the following line

AUDIT_SPLIT.AGED_CUM_30 > 0 AND SALES_LEDGER.RESTRICT_MAIL = 0  and COMMUNICATION_ADDRESS.LETTER_TYPE_ID = 1

What I think you need to do is change the CUM_30 > 0  to be  <> 0

It worked for me with a customer who had a credit balance.