This can be used just for Washington State, as long as you have the tax-by-zip code.

The 'advantage' of this system is that it creates a new zip tax table and uses that to look up the rates. 

In the admin you will have an area to import the csv file into oscommerce

Requirements:

The Sales Tax GIS data available for download on this page is for Quarter 2, 2008 (Effective April 1, 2008) and effective for the entire state. The downloadable data below is provided as a resource for you to integrate into your own customized applications and for your research purposes. The files are in comma-separated (.txt) format, in zip files.

Data:  http://dor.wa.gov/content/FindTaxesAndRates/SalesAndUseTaxRates/stdownloads.aspx
 

Tax By Zip Code for Washington - Oscommerce Contribution Installation