Can I add "balance due" to Sales by Customer Detail Report?

The Sales by Customer Detail does have Balance column and I think there is an "OPEN BALANCE" checkbox you can select (select the gear option for the report).  Not sure if this will give you the info you are looking for but it might be helpful.  

Hey there groundcontrol-na, 

It's always a pleasure to see new faces in the Community, Welcome! QuickBooks Online offers various reports that can help you better understand your finances. With reports, you have the option to filter as needed. To view Open balance in the Sales by Customer Detail report, you can click on the small Gear icon and select Open balances. You'll be able to add/remove checkmarks as you see fit. 

QuickBooks Online also provides the option to create a Customer statement, which shows you any outstanding balances for specific customers you chose. If this is something that interests you, this helpful guide will provide the steps on how you can create a customer statement. 

If you have any other questions, feel free to reach back out to us here.