public function tax(Product $product)
    {
        $tax = $this->money($product);

        if (! $product->taxable || $product->freebie) {
            return $tax;
        }

        $value    = $this->value($product);
        $discount = $this->discount($product);

        $value = $value->subtract($discount);
        $tax   = $value->multiply($product->rate->float());

        return $tax;
    }