public function add_fee( $args = array() ) {
		$fee_props            = (object) wp_parse_args( $args, $this->default_fee_props );
		$fee_props->name      = $fee_props->name ? $fee_props->name : __( 'Fee', 'woocommerce' );
		$fee_props->tax_class = in_array( $fee_props->tax_class, array_merge( WC_Tax::get_tax_classes(), WC_Tax::get_tax_class_slugs() ), true ) ? $fee_props->tax_class : '';
		$fee_props->taxable   = wc_string_to_bool( $fee_props->taxable );
		$fee_props->amount    = wc_format_decimal( $fee_props->amount );

		if ( empty( $fee_props->id ) ) {
			$fee_props->id = $this->generate_id( $fee_props );
		}

		if ( array_key_exists( $fee_props->id, $this->fees ) ) {
			return new WP_Error( 'fee_exists', __( 'Fee has already been added.', 'woocommerce' ) );
		}

		$this->fees[ $fee_props->id ] = $fee_props;

		return $this->fees[ $fee_props->id ];
	}