Let's set up late fees together so you can charge your customers accordingly, Charlie.

When applying late fees in QuickBooks Online (QBO), the system will only calculate and apply to your invoices the day after you turn on the late fee setting. Since you want to add it to your old invoices, I'd suggest adding it manually by following the steps above.

The option to add a percentage to products and services is unavailable. As a workaround, you can add a flat rate for that amount and calculate the equivalent amount of the 1.5% late fee you want.

Furthermore, refer to this article for a step-by-step guide on manually adding late fees to your old invoices: Apply late fees to overdue invoices automatically in QBO.

Additionally, you can streamline your invoicing process by enabling automatic reminders for unpaid invoices, ensuring timely payments and improved cash flow for your business.

Feel free to comment if you still have concerns about late payments on your invoices. I'm here to help you.