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



