The Cloud Cart Connector can sync orders from QuickBooks Online into ShipStation. Here are the steps:
1. Set-up a scheduled task with Cloud Cart Connector
2. Turn on QuickBooks class tracking
3. Create some new invoices or sales receipts in QuickBooks
4. Ensure these transactions are formatted properly.
This video demonstrates these steps.
For more information read this KB: Sync QuickBooks Online to ShipStation