Total due on the account can either be calculated from the total of all open documents (open invoices, unallocated payments, credit memos, etc.) or displayed as it is received from your ERP system.

Your accounts may be configured to display either Calculated Customer Balance or ERP Balance. This can be configured using the "Use calculated customer balance" setting that can be found on the Settings > Business page. Please ask an administrator if you do not have access to these settings.

So, if you are using ERP Balance and see that the total of the invoices does not match this ERP balance value, it could happen because of the following reasons:

1. ERP Balance field may take into account, open credit memos, open payments, and journal entries, whereas you only take into account open invoices. Because of that, we may see a difference between the sum of all invoices and the total due on the customer record. Take a look at the Open Credits and Adjustments tabs to make sure everything is in place. 
2. Some documents may not be flowing into YayPay due to the integration issue, for example, an invoice was already paid, and the account ERP balance is 0, but there's still an open invoice in the list of invoices. 

If you use a Calculated customer balance but the total of the open documents does not match this value - please, let us know to investigate further.