^^&&^^

Edit File: class-kkart-shipping-flat-rate.php

<?php
/**
 * Flat Rate Shipping Method.
 *
 * @version 2.6.0
 * @package Kkart\Classes\Shipping
 */

defined( 'ABSPATH' ) || exit;

/**
 * KKART_Shipping_Flat_Rate class.
 */
class KKART_Shipping_Flat_Rate extends KKART_Shipping_Method {

/**
	 * Cost passed to [fee] shortcode.
	 *
	 * @var string Cost.
	 */
	protected $fee_cost = '';

/**
	 * Constructor.
	 *
	 * @param int $instance_id Shipping method instance ID.
	 */
	public function __construct( $instance_id = 0 ) {
		$this->id                 = 'flat_rate';
		$this->instance_id        = absint( $instance_id );
		$this->method_title       = __( 'Flat rate', 'kkart' );
		$this->method_description = __( 'Lets you charge a fixed rate for shipping.', 'kkart' );
		$this->supports           = array(
			'shipping-zones',
			'instance-settings',
			'instance-settings-modal',
		);
		$this->init();

add_action( 'kkart_update_options_shipping_' . $this->id, array( $this, 'process_admin_options' ) );
	}

/**
	 * Init user set variables.
	 */
	public function init() {
		$this->instance_form_fields = include __DIR__ . '/includes/settings-flat-rate.php';
		$this->title                = $this->get_option( 'title' );
		$this->tax_status           = $this->get_option( 'tax_status' );
		$this->cost                 = $this->get_option( 'cost' );
		$this->type                 = $this->get_option( 'type', 'class' );
	}

/**
	 * Evaluate a cost from a sum/string.
	 *
	 * @param  string $sum Sum of shipping.
	 * @param  array  $args Args, must contain `cost` and `qty` keys. Having `array()` as default is for back compat reasons.
	 * @return string
	 */
	protected function evaluate_cost( $sum, $args = array() ) {
		// Add warning for subclasses.
		if ( ! is_array( $args ) || ! array_key_exists( 'qty', $args ) || ! array_key_exists( 'cost', $args ) ) {
			kkart_doing_it_wrong( __FUNCTION__, '$args must contain `cost` and `qty` keys.', '4.0.1' );
		}

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

// Allow 3rd parties to process shipping cost arguments.
		$args           = apply_filters( 'kkart_evaluate_shipping_cost_args', $args, $sum, $this );
		$locale         = localeconv();
		$decimals       = array( kkart_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 ? KKART_Eval_Math::evaluate( $sum ) : 0;
	}

/**
	 * Work out fee (shortcode).
	 *
	 * @param  array $atts Attributes.
	 * @return string
	 */
	public function fee( $atts ) {
		$atts = shortcode_atts(
			array(
				'percent' => '',
				'min_fee' => '',
				'max_fee' => '',
			),
			$atts,
			'fee'
		);

$calculated_fee = 0;

if ( $atts['percent'] ) {
			$calculated_fee = $this->fee_cost * ( floatval( $atts['percent'] ) / 100 );
		}

if ( $atts['min_fee'] && $calculated_fee < $atts['min_fee'] ) {
			$calculated_fee = $atts['min_fee'];
		}

if ( $atts['max_fee'] && $calculated_fee > $atts['max_fee'] ) {
			$calculated_fee = $atts['max_fee'];
		}

return $calculated_fee;
	}

/**
	 * Calculate the shipping costs.
	 *
	 * @param array $package Package of items from cart.
	 */
	public function calculate_shipping( $package = array() ) {
		$rate = array(
			'id'      => $this->get_rate_id(),
			'label'   => $this->title,
			'cost'    => 0,
			'package' => $package,
		);

// Calculate the costs.
		$has_costs = false; // True when a cost is set. False if all costs are blank strings.
		$cost      = $this->get_option( 'cost' );

if ( '' !== $cost ) {
			$has_costs    = true;
			$rate['cost'] = $this->evaluate_cost(
				$cost,
				array(
					'qty'  => $this->get_package_item_qty( $package ),
					'cost' => $package['contents_cost'],
				)
			);
		}

// Add shipping class costs.
		$shipping_classes = KKART()->shipping()->get_shipping_classes();

if ( ! empty( $shipping_classes ) ) {
			$found_shipping_classes = $this->find_shipping_classes( $package );
			$highest_class_cost     = 0;

foreach ( $found_shipping_classes as $shipping_class => $products ) {
				// Also handles BW compatibility when slugs were used instead of ids.
				$shipping_class_term = get_term_by( 'slug', $shipping_class, 'product_shipping_class' );
				$class_cost_string   = $shipping_class_term && $shipping_class_term->term_id ? $this->get_option( 'class_cost_' . $shipping_class_term->term_id, $this->get_option( 'class_cost_' . $shipping_class, '' ) ) : $this->get_option( 'no_class_cost', '' );

if ( '' === $class_cost_string ) {
					continue;
				}

$has_costs  = true;
				$class_cost = $this->evaluate_cost(
					$class_cost_string,
					array(
						'qty'  => array_sum( wp_list_pluck( $products, 'quantity' ) ),
						'cost' => array_sum( wp_list_pluck( $products, 'line_total' ) ),
					)
				);

if ( 'class' === $this->type ) {
					$rate['cost'] += $class_cost;
				} else {
					$highest_class_cost = $class_cost > $highest_class_cost ? $class_cost : $highest_class_cost;
				}
			}

if ( 'order' === $this->type && $highest_class_cost ) {
				$rate['cost'] += $highest_class_cost;
			}
		}

if ( $has_costs ) {
			$this->add_rate( $rate );
		}

/**
		 * Developers can add additional flat rates based on this one via this action since @version 2.4.
		 *
		 * Previously there were (overly complex) options to add additional rates however this was not user.
		 * friendly and goes against what Flat Rate Shipping was originally intended for.
		 */
		do_action( 'kkart_' . $this->id . '_shipping_add_rate', $this, $rate );
	}

/**
	 * Get items in package.
	 *
	 * @param  array $package Package of items from cart.
	 * @return int
	 */
	public function get_package_item_qty( $package ) {
		$total_quantity = 0;
		foreach ( $package['contents'] as $item_id => $values ) {
			if ( $values['quantity'] > 0 && $values['data']->needs_shipping() ) {
				$total_quantity += $values['quantity'];
			}
		}
		return $total_quantity;
	}

/**
	 * Finds and returns shipping classes and the products with said class.
	 *
	 * @param mixed $package Package of items from cart.
	 * @return array
	 */
	public function find_shipping_classes( $package ) {
		$found_shipping_classes = array();

foreach ( $package['contents'] as $item_id => $values ) {
			if ( $values['data']->needs_shipping() ) {
				$found_class = $values['data']->get_shipping_class();

if ( ! isset( $found_shipping_classes[ $found_class ] ) ) {
					$found_shipping_classes[ $found_class ] = array();
				}

$found_shipping_classes[ $found_class ][ $item_id ] = $values;
			}
		}

return $found_shipping_classes;
	}

/**
	 * Sanitize the cost field.
	 *
	 * @since 3.4.0
	 * @param string $value Unsanitized value.
	 * @throws Exception Last error triggered.
	 * @return string
	 */
	public function sanitize_cost( $value ) {
		$value = is_null( $value ) ? '' : $value;
		$value = wp_kses_post( trim( wp_unslash( $value ) ) );
		$value = str_replace( array( get_kkart_currency_symbol(), html_entity_decode( get_kkart_currency_symbol() ) ), '', $value );
		// Thrown an error on the front end if the evaluate_cost will fail.
		$dummy_cost = $this->evaluate_cost(
			$value,
			array(
				'cost' => 1,
				'qty'  => 1,
			)
		);
		if ( false === $dummy_cost ) {
			throw new Exception( KKART_Eval_Math::$last_error );
		}
		return $value;
	}
}