Sales Tax Web Service Look up

[dacor]

I have searched the boards and FAQ's and have not found the answer for this one. I don't know about the other states, but in Washington state sales tax is based on the address of delivery of goods. And in this state there could be 4 different tax rate in a four block area. Everything is based on zip+4. The nice thing is that the state has developed a simple web service to find the tax rate for any location.
example; http://dor.wa.gov/AddressRates.aspx?...&zip=985011001
response: LocationCode=3406 Rate=0.089 ResultCode=0
I've looked at several plugins for the sales tax and they all have significant cost or issues that make this a much cleaner solution. I want to find where the shopping cart pulls the percentage for the defined zone rate (in my case washington state zone) and overwrite it with this new LocationCode value. I have taken a first look at the code and see a ton of files to wade through. I think I will add a function to return the rate and overwrite the sql value. Does anyone know which files pull the tax rate for the shopping cart?
Thanks... daveo

[kobra]

You can start with
includes/classes/order.php and about line 41

[RixStix]

Tax Cloud is also a free option.

https://taxcloud.net/#pricing

[dacor]

Tax cloud is free however, they want to pay the tax to the state. THis will not work for us since we do off line sales as well.