Please go and read the legal text from the middle of:
https://secure.shippingapis.com/registration/
It's the US Postal Services legal agreement for use of its web API. Did you know that it specifically prohibits you from using the sample code they provide? "The sample code, the documentation, and the trademarks/logos provided on this site and in hardcopy form are the intellectual property of USPS protected under U.S. laws. The information and images presented may not be reproduced, republished, adopted, used, or modified under any circumstances."
Why would you publish sample code and then prohibit it's use?
But it gets even more amazing: In that same agreement they say: "User agrees to use the USPS Web site, APIs and USPS data to facilitate USPS shipping transactions only." So, for example, you can NOT look up a zip code to ship a package via UPS.
And they will seriously try to verify that you use this ONLY for USPS. I called and asked and was told that I could not even use the API in a system where the customer MIGHT pick another service. The guy I talked to was deadly serious that if I used that API to correct an address and then I shipped the package via FEDEX, I would be violating the contract.
I assume rates would be the same since they also come from the API. E.g. you can not display competitive rates. Lots of people do, of course, but I wonder if they actually use this API or do they get thier rates from some other source? USPS lists thier rates on the web site, so why not just get them from that and build a table?
Even that very resonable idea appears to be restricted by thier terms. If you go to the mail USPS web site, and check the "terms of use" at the bottom, you find:
"The information and images presented here may not under any circumstances be reproduced or used without prior written permission. Users may view and download material from this site only for the following purposes: (a) for personal, non-commercial home use; (b) where the materials clearly state that these materials may be copied and reproduced according to the terms stated in those particular pages; or (c) with the express written permission of the Postal Service."
In the rate sections, there are no statements that say you can reproduce the information. So unless it is for your "personal, non-commercial home use" you are prohibited from building a shipping table manually.
IANAL, but if "terms of use" has any legal force, every person who ever copied information from the USPS web site to thier shopping cart shipping cost table is violating a contract with a US federal agency.
And it isn't just the USPS; check the legal text of the FedEx and UPS API's:
UPS: "The UPS Systems and Information are to be used solely by Access User in connection with shipments tendered by, to or for Access User to UPS for delivery and for no other purpose. "
http://www.ups.com/content/us/en/bussol/offering/technology/alliances/application_erp/terms.html
FEDEX: "You will not incorporate in an Application an automated comparison feature which compares FedEx rates and services with those of other carriers."
(I can't provide a link to that one because you must be logged into your FedEx account to access it.)
"Use of fedex.com to provide information to or prepare shipments by or for the benefit of third party shippers is expressly prohibited."
http://www.fedex.com/cgi-bin/global_legal.cgi?cc=us&sc=/legal/
Anyone who has registered for access to the USPS, UPS, or FedEx API systems, and who later displays on thier ecommerce site, any comparison of rates between those services, is in fact violating the agreement they made for access to those APIs.