CODE
class Product extends ProductCore
{
    const US_COUNTRY_ID = 21;
    const RATE = 0.10;

    public static function getPriceStatic($id_product, $usetax = true, $id_product_attribute = null, $decimals = 6, $divisor = null,
        $only_reduc = false, $usereduc = true, $quantity = 1, $force_associated_tax = false, $id_customer = null, $id_cart = null,
        $id_address = null, &$specific_price_output = null, $with_ecotax = true, $use_group_reduction = true, $context = null,
        $use_customer_price = true, $id_customization = null)
    {
        $id_cart = ($id_cart == null || $id_cart == '') ? 0 : (int)$id_cart;

        $price = parent::getPriceStatic($id_product, $usetax, $id_product_attribute, $decimals, $divisor,
            $only_reduc, $usereduc, $quantity, $force_associated_tax, $id_customer, $id_cart,
            $id_address, $specific_price_output, $with_ecotax, $use_group_reduction, $context,
            $use_customer_price, $id_customization);

        $cart = new Cart($id_cart);
        if(Validate::isLoadedObject($cart) && $this->isUsDeliveryCart($cart)) {
            $price = (float)Tools::ps_round($price * self::RATE, 2);
        }

        return $price;
    }

    /**
     * Determine whether the cart's DELIVERY address is in the USA.
     */
    protected function isUsDeliveryCart($cart)
    {
        if (!Validate::isLoadedObject($cart) || !(int)$cart->id_address_delivery) {
            return false;
        }

        $address = new Address((int)$cart->id_address_delivery);
        return Validate::isLoadedObject($address)
            && (int)$address->id_country === (int)self::US_COUNTRY_ID;
    }
}