ELEX Stamps.com Plugin – Authentication Failed Error

Customer:

I am using the stamps.com plugin with WooCommerce in WordPress. It is installed correctly, but when I go to calculate shipping I get the following error:
Stamps.com – Unable to Get Auth: Authentication failed.

I am pretty sure I have the correct UN and PW because I can use it actually to get into my stamps.com acct.

Please help.

Support:

If the authentication information like UserID or Password is not correct, you get this Auth: Authentication failed error.

We request you to check the UserID and Password once again. For more information on troubleshooting, please refer the Stamps.com troubleshooting article.