You need to sign up for a USPS Webtools account, it will not work with a normal USPS account.
Go to
http://www.usps.com/webtools/ or the signup page
https://secure.shippingapis.com/registration/
There are two usernames and passwords required to get shipping working. One is for your Globecharge account and the other is for Webtools USPS. The right one has to be added in the right location.
Another thing is that if you use USPS you need to make sure that you use weights in your products.
Also in your payment settings, Properties => Payment methods accepted you also need to specify your GlobeCharge username there.