public function addProduct(
        $id,
        $description,
        $productType,
        $price,
        $quantity,
        $vatCode,
        $vatPercentage
    ) {
        if (! is_numeric($price)) {
            throw new Error('Price must be numeric', 1);
        }
        if (! is_numeric($quantity)) {
            throw new Error('Quantity must be numeric', 1);
        }

        $this->_products[] = array(
            'productId'     => $id,
            'productType'   => $productType,
            //description mag maar 45 chars lang zijn
            'description'   => substr($description, 0, 45),
            'price'         => $price,
            'quantity'      => $quantity * 1,
            'vatCode'       => $vatCode,
            'vatPercentage' => $vatPercentage
        );
    }