Set a Default Payment Method for ShipStation Order Import
It is recommended you create a payment method in QuickBooks that is an "Other Credit Card" type. Assign this payment method in the Order Import tab of the ShipStation configuration.
You can assign a Default Payment Method on the Order Import tab of the ShipStation integration at File > Configuration Management > Services > ShipStation > Order Import:
How to create a Payment Method in QuickBooks.
Please review our Docs page for additional ShipStation configuration settings.