This is part programming, but I think people who may have had a similar problem can help, and even if you haven't, I'm sure someone here can help. Right now, we are using osCommerce, and it has a USPS shipping module. That requires a User ID which we have and a pass which is doesn't give you. I am told that you just leave it blank or make it the user ID. But every time I do, it I keep getting an error: An error occurred with the USPS shipping calculations. If you prefer to use USPS as your shipping method, please contact the store owner. So, how do I fix that? It's shipping, so... it's important.
Why not take a look for the contribution page of oscommerce? I believe you should use the latest shipping modules.