You need to sign up for a USPS WebTools account, it will not work with a normal USPS account.

USPS Web Tools APIs enable external e-commerce shopping cart and shipping software to access valuable USPS data such as shipping rates.
Go to http://www.usps.com/webtools/
or their signup page
https://registration.shippingapis.com/

Once you have signed up for a USPS WebTools account, you can setup your shop to use USPS shipping provider.
Note: you need to make sure that you enter correct weights in your products so that the shipping cost is calculated properly. If 0.00 weight - then shipping cost is $0.00.
- Open your shop in ShopFactory

- Click on Properties -> Shipping Charges -> Use a Provider -> Click Next.

- Select "Combine multiple shipping providers" option even if you only want to setup one (1) shipping provider i.e. USPS. This interface is more up to date which is why we recommend using this option. UPS, FedEx, Australia Post, Royal Mail, PostNL providers can also be setup using this option.

- Click Next.

- Enter your shop details i.e. ZipCode and weight unit = Pounds (lbs)

- Enter your USPS Login and USPS password

- Select the shipping methods you want to enable.

- Click OK.
ShopFactory cannot check if your USPS WebTools username/password is correct, as this is given to you by USPS. You need to contact them to find out if the username/password if valid.