In an earlier version of BookingLive, an issue arose where Event Upsells required a Resource for them to work properly. Since then, this issue has been resolved, but this has had an unintended side effect due to a new feature:

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You can link a Resource to an Upsell, to limit the maximum number of that upsell that can be sold. This then shows as a quantity selector, and the number of that upsell that is available.

If you do not wish to limit the upsell, and want the upsell to just be a checkbox, then you will need to go into the Upsell product, then go to the Resources Tab, and then unlink the Resource:

This will now display the Upsell as a simple Checkbox:

In version 3.7, there is a new feature, which will allow you to define the minimum and maximum number of times an upsell can be added per order. You will still be able to link the upsells to a Resource to provide an overall capacity for the upsell across multiple products, or for date-specific upsell options.