An order bump sits on the checkout the buyer is already completing. The main offer is decided. The bump asks a smaller yes: the workbook, the template pack, the office hours. It should be obviously related. A bump that feels like a different business teaches the buyer not to trust the page.
On RobinCart the bump can fulfil on its own. If the checkbox opens a second Thinkific course, map that course to the bump, not to the main product, and refund-test both.