How do I use USPS Priority Flat Rate?

Explains how to create a label for a Priority Mail Flat Rate shipment in ShipStation.

You can apply USPS Priority Flat Rate to your shipments in ShipStation. To use this service, you must do two things:

  • Enable Package types for Flat Rate options in ShipStation, and

  • Select Flat Rate as the Package type.

Flat Rate packaging will only be available when you select USPS Priority as the Service.

The service drop-down is expanded in the Configure Shipment Widget and USPS Priority Mail is selected as the service.

Then, as long as you have enabled Flat Rate Package types in ShipStation, they will show in the Package drop-down to select when you configure your shipments.

The packages drop-down is expanded and all of the flat rate package options are highlighted.

See the USPS Priority Mail page for more information and restrictions. Additional fees may apply when you add certain shipping options.


If you try to use the Flat Rate package with a USPS service other than Priority Mail, you’ll get an error. These package types can only be used with Priority.