How do I charge tax using WordPress Simple Paypal Shopping Cart?

I’ve searched and searched and cannot figure out how to charge tax for physical purchases using wordpress Simple Paypal shoppint Cart. I know that it get integrated with Paypal somehow but don’t know where to get information on how to do this.

Can someone please share how to charge tax using this plugin?

Thanks

http://wordpress.org/plugins/wordpress-simple-paypal-shopping-cart/