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You can find the updated Miva 10 Reference Guide here
Use this tab to automatically add sales tax to orders based on the customer's state. Please note that, by default, Miva Merchant can only apply a sales tax rate to an entire state. If you need to apply sales tax in a more sophisticated way, you must purchase a third party module.
Click Save Record
When a customer places an order from California, the sales tax you created for that state will automatically be applied to the order. The sales tax will appear in the Payment Information screen and on the invoice.
Note that, as of version 9.0004, Miva Merchant supports three decimal places for the tax rate. Previous releases of Miva Merchant supported two decimal places in the Rate field.