Normally, products price (and tax class) are added throug easypopulate.php.
On a shop in development the products have multiple prices (per size). These come from the products_plant table and are stored in the plant_price column.
This table is filled through the [login to view URL] script.
That script doesn't take count of the configured taxes in oscommerce (or the modified oscommerce that is used).
This needs to be changed so that it processes the taxes.