Fulfillment holds

When upsell offers are presented to customers at checkout, the order fulfillment status is set to On hold until the customer completes their order. When an order is On hold you can reserve inventory for the order, but you can't fulfill the order until the fulfillment hold is released, and the order fulfillment status changes to Unfulfilled.

If a customer doesn't complete a checkout, then the order status remains On hold for one hour to allow the customer time to return to the order and complete their checkout.

Please note:

This process is not app related. It's managed by Shopify. You'll get the same result with any other post purchase one click app.