USPS credentials not accepted

I’ve downloaded the USPS Live Rates plugin and can’t get past the general settings. When I enter my USPS.com user ID, I keep getting the “Authorization failure. Perhaps username and/or password is incorrect” message.

I’ve entered the USPS username I’ve had for years, and also made a new one with a new email address. Neither of them worked after several tries. I tried both the username and the email address for both accounts.

Also the error message is confusing. Am I supposed to be entering my password along with my username??

Some clarity would be appreciated.