I understand the fixed amount of sales tax per state that your shopping cart offers. But the sales tax in Washington State is called "Destination based sales tax" meaning the sales tax is calculated based on the destination of the shipment. Each zip code has a different tax rate. Here is the DOR's explanation OnlineSellers.aspx Its not just one flat rate for the entire state. I'm looking for a shopping cart that can calculate "Destination based sales tax"?