Hi,
In order to calculate the shipping fees, USPS requires the destination address. Before registering, the user didn't enter his address. So of course, USPS cannot give any fees.
Regarding the fact that the prices are different, it can come from different things:
-> Did you test with the exact same addresses ?
-> Sometimes the costs offered by USPS may vary between different customers (of USPS). Again, they can be different from the default prices. So when you test on the USPS website, make sure that they are sending you your rates, not the rates everyone is getting by default.
-> There are other options to the USPS plugin like the "group package" option or the "include prices" option which could increase the shipping fees.
-> Check that the main weight unit (the first one) of the configuration is the same as the one you're using for your products and shipping method (check the dimension too).