Can I let customers choose only one option in a product add-on? (Essential Upsell app)

Not at the moment. In a Product Add-on, each option is an independent checkbox, and there is no setting for a maximum number of selections or a choose-one mode. Customers can select more than one add-on from the list.

Two settings look similar but do something different:

• Limit discounted product quantity limits how many units receive the discount. It does not limit how many different add-ons a customer can select. Setting the discounted quantity to 1 does not create a choose-one option.
• Show quantity picker lets the customer choose a quantity for each selected product.

If you need a choose-one flow, contact support and describe the setup you want.