Another add-on for the Concrete5 eCommerce package,my USPS Shipping Module allows you to retrieve worldwide shipping rates and service information for the United States Postal Service.

You'd figure that if you've already written shipping module for UPS it should be pretty easy to write another one for the United States Postal Service, wouldn't you?  They do after all both use a similar XML API and provide the same kind of functionality, right?  Yes, they do, but remember it's never safe to make assumptions.

The USPS shipping module turned out to require probably twice as much time to write as the UPS module. That was in spite of the fact that much of the code in the UPS module was reused in the USPS module. Why is that? Well there are a number of reasons, some of which I won't repeat because my mother taught me not to say bad things about other people (or governmental organizations, in this case).  The main reason, though, is that the USPS provides many more shipping services than the UPS. The USPS is also distinguished in that the rules they use for determining what sizes of packaging, what weights are permissible, and a multitude of other factors are far more complicated than the rules the UPS uses.

Please take a look at the USPS Shipping Module for more details.