Hi Mary Bosman 

Currently, there is no built-in option to add interest on outstanding invoices. To manage this, you'll need to manually adjust the customer account to include the interest balance.

Here are the steps to do this:

1. Navigate to Customers.
2. Select Transactions.
3. Choose Customer Adjustment.

For detailed instructions, you can refer to this article: Sage Knowledge Base.

Once done please confirm if the answer provided did help by clicking on verify answer.

Sharon