Hi Cooper's Handcrafted Soap Co - 

ShippingEasy provides the option to select USPS Flat Rate shipping options for both Priority Mail and Priority Mail Express. To access these, you'll first need to register an Endicia Expedited account. This allows us to quote rates for USPS expedited services from Endicia's label server.

Please use this guide to help you open the account:

• How to: Open a new Endicia (USPS) account for free

Learn more about the difference between your two Endicia accounts.

Once you've registered the account, you can select USPS Flat Rate shipping:

1. Create a new shipment for an order. 
2. From the Ready to Ship page, toggle the Carrier to USPS, the Service to Priority or Express Mail, the Package to the Flat Rate option you see.
3. The rate quote will update. Then you can purchase the label.

I hope this helps! Please post your follow up questions in the comments below. Thanks!