Testing Real Time Shipping Methods
Your EasyPost account can be in two modes, live and test. When testing, be sure to put your account in Test Mode.
Click on Dashboard and you will see a toggle there as seen below.
From there click on Account > Carriers and you can add additional test carriers there.
To test each carrier, you will need to create a live account. Just follow the prompts for each one to sign up. Once you add each new carrier they will show as expected at checkout.