If you want an upsell to be automatically checked by default when a customer selects an offer, go to the bundle editor, open the Upsells tab, then:
Click on the upsell you want to set as default.
Scroll down to Select by default.
Enable this option.
Result: as soon as a user lands on the product page, the upsell will already be selected (as long as it’s linked to the selected offer).
Important:
If multiple upsells have Select by default enabled, they will all be checked by default.
If the upsell doesn’t appear or isn’t selected automatically, make sure it’s linked to the correct offer via Match To, and that the product is properly selected.
If you run into any issues, contact MoonBundles support.