You can choose to send the confirmation when Upsells Start or when Upsells End.

Behavior can be further customized for different cases:

• When upsell orders are to be merged with original order (default: Upsells End)
• When upsell orders are to be created as a separate order (default: Upsells Start)
• When upsell orders are created as a separate order and Primary Order Is Cancelled (default: Upsells End)

Once you've made all the changes, click on Save.