Customize Virtuemart for multiple tax rates...

I need to customize our virtuemart installation a little bit. We're US based, and no configurations work unless the PHP code is modified. We don't expect to have any sales outside of the US. We need to have multiple tax rates- some items have no tax, some have 8%. Yes, it may be as simple as turning on EU mode, and adding 'USA' to the list of European countries in the [login to view URL] file, but there's more... Those tax rates should only be effective if the ship-to address is in California- everyone else should not be taxed. Obviously when people add an item to their cart, they should see the prices without tax until they actually press "Check-Out", and the system asks for their address. Then the system should calculate whether or not they should be taxed. To be more clear, adding a product to the cart in Virtuemart shows you the products that are added, along with the tax, the subtotal, and the grand total. That screen should only have their product(s) and the sub-total. Joomla assumes that people want to see the price with tax, and assumes that you're in a taxable state until check out. That's counter-intuitive, and simply makes the price seem higher, so when people click on the main product from the category, and then choose a child product, we need the prices that they see to be sans tax (ie., "Item #1 $80", NOT "Item #1 $86.26") All changes to the core files should have comments in the code noting the addition of new code, and old code should only be commented out, not removed. We are running Joomla 1.5.3, and VirtueMart 1.1.3 on an Apache web-server running PHP 5.2.6. It is a standard installation of Virtuemart, and with the exception of ps_product, where I modified the code to allow product files to be featured; and [login to view URL], where I modified the code to make child products appear in order. Both of those files are attached, should they be necessary at all.