protected function evaluate_cost( $sum, $args = array() ) {
		// Add warning for subclasses.
		if ( ! is_array( $args ) || ! array_key_exists( 'qty', $args ) || ! array_key_exists( 'cost', $args ) ) {
			wc_doing_it_wrong( __FUNCTION__, '$args must contain `cost` and `qty` keys.', '4.0.1' );
		}

		include_once WC()->plugin_path() . '/includes/libraries/class-wc-eval-math.php';

		// Allow 3rd parties to process shipping cost arguments.
		$args           = apply_filters( 'woocommerce_evaluate_shipping_cost_args', $args, $sum, $this );
		$locale         = localeconv();
		$decimals       = array( wc_get_price_decimal_separator(), $locale['decimal_point'], $locale['mon_decimal_point'], ',' );
		$this->fee_cost = $args['cost'];

		// Expand shortcodes.
		add_shortcode( 'fee', array( $this, 'fee' ) );

		$sum = do_shortcode(
			str_replace(
				array(
					'[qty]',
					'[cost]',
				),
				array(
					$args['qty'],
					$args['cost'],
				),
				$sum
			)
		);

		remove_shortcode( 'fee', array( $this, 'fee' ) );

		// Remove whitespace from string.
		$sum = preg_replace( '/\s+/', '', $sum );

		// Remove locale from string.
		$sum = str_replace( $decimals, '.', $sum );

		// Trim invalid start/end characters.
		$sum = rtrim( ltrim( $sum, "\t\n\r\0\x0B+*/" ), "\t\n\r\0\x0B+-*/" );

		// Do the math.
		return $sum ? WC_Eval_Math::evaluate( $sum ) : 0;
	}