Adding late payment charge

Hello and welcome to the Community, ruthie-mildwater

There's no feature for adding automated interest in QuickBooks Online, but you can add this manually by following the steps below:

1. Go to the Gear icon and choose Products and Services.
2. Click on New and select Service.
3. Enter the name of the item and choose an income account.
4. You can also enter the rate of the item.
5. Click on Save and close once you're done.

Once created, you can then add this as a new item line on the invoice.