1 penny tax difference issue on every subsequent product that is added to cart.
Example if 10 of the same product is added than the difference is 5 cent and not penny, the more the product the more the difference.
In shopper group 'Show Prices including tax' is checked.
There is a one penny difference in tax calculation when there are more than two products in shopping cart, the reason instead of tax being calculated on total price of the products it is calculated for individual product rounded and added to total, for better explanation see example below.
In this example tax is 9.75% (0.0975) and product cost $10.
$10 + 9.75%Tax = $10.975 which VirtueMart rounds it to $10.98
When 2 quantity or more is ordered VirtueMart adds the total so it is $10.98 + 10.98 = $21.96 which is Wrong should be
$10 + $10 = $20 + 9.75% = $21.95
However adding this way is wrong too, because what if one of the products doesn’t have tax so best solution would be not to round the tax until it is added to the cart.
$10.975 + $10.975 = $21.95 instead of
$10.98 + $10.98 = $21.96This needs to be fixed, you might think 1penny is nothing but if more quantity is added than the difference adds up and if customer notices this, than store owner has to issue a refund, which result to more fee (credit card refund fees)


Some information about tax calculation
http://docs.phpcoin.com/index.php/Tax_Calculations