If your customer has outstanding balances, but their invoices are showing it is $0.00 it could be because their invoices have not been sent or at least marked sent.
To this this you will click the “Finance” tab and click “Invoices”
You will then select the box beside the invoice you’re wanting to mark sent and click “mark as” and select how it was sent.
The outstanding balance should automatically update with any totals not paid from the sent invoices.
(Unless the box to “hide outstanding balances” is selected in your settings)