Hello PASC,

Thank you for connecting to the QuickBooks Community! For this situation, you would need to manually add this interest as a line item on the older invoices. Regarding getting the compound interest charge, that would also need to be calculated separately. Please use the steps link below to assist with creating a line item.

1. Go to Lists, then select Item List (for Windows) or Items (for Mac).
2. Select Item then New (for Windows) or plus + (for Mac), then select New Item.
3. Select the type of item you want to create.
4. Fill out the item fields.
5. Use Custom Fields to add your own customized fields.
6. Select Save.

Let me know if you have additional questions. I'm more than happy to help. See you soon.