I got a web account number and tried setting up this module and when you checkout on the store it says under USPS:

We are unable to find a USPS shipping quote suitable for your mailing address and the shipping methods we typically use.
If you prefer to use USPS as your shipping method, please contact us for assistance.
(Please check that your Zip Code is entered correctly.)

This is how I configured it on zen cart:

Enable USPS Shipping
True

Enter the USPS Web Tools User ID
My code

Which server to use
production

Handling Fee
2

Tax Class
Taxable Goods

Tax Basis
Shipping

Shipping Zone
--none--

Sort Order
0

All Packages are Machinable
False

USPS Options
Display weight, Display transit time

Domestic Shipping Methods
Express, Priority

Int'l Shipping Methods