When a customer adds an upsell through the post-purchase offer presented by the Last Upsell app, the added item(s) extends the existing order. The order can be edited only when it's still in an unfulfilled state. There is no new order created.
We are using an Edit Order API to alter the existing order and add any new items. This is how it looks from the order timeline perspective. The third step (auto-removing unpaid upsell) is adjustable via the app settings.