I'm wondering if anybody here has used a web service with their E-commerce site for calculating sales tax based on the buyer's zip code, etc. I know there are web services out there like
http://www.strikeiron.com/productdetail.aspx?p=351
and
http://www.serviceobjects.com/products/dots_fasttax.asp
and they both seem to provide sample PHP/Ruby/ASP/Java-based code for integrating with shopping cart solutions and e-commerce based sites. Have you used any such services? If so, what's missing from these services today that needs to be there? Would you see yourself using such a service?

Basically, I am trying to figure out if making a competing web service and PHP/Ruby/ASP/Java libraries that provides sales tax data with real-time web lookups would be valuable to web programmers. I'm also assuming that our service could be cheaper to these ones above.

Any thoughts based on your experience would be highly appreciated.
Thanks everyone!