Snippet to change cutoff time based on shipping method.PluginHive Plugins : https://www.pluginhive.com/plugins/

functions.php

/**
 * Snippet to change cutoff time based on shipping method.
 * Created at : 28 Nov 2018
 * PluginHive Plugins : https://www.pluginhive.com/plugins/
 */
add_filter( 'ph_estimated_delivery_get_cutoff_time', 'ph_estimated_delivery_get_custom_cutoff_time', 10, 2 );
if( ! function_exists('ph_estimated_delivery_get_custom_cutoff_time') ) {
	function ph_estimated_delivery_get_custom_cutoff_time( $cutoff, $cur_day ) {	
		$adjustment = array(
								'table_rate:2:1'=>array( 'mon'=>"18:00", 'tue'=>"18:00", 'wed'=>"18:00", 'thu'=>"18:00", 'fri'=>"18:00", 'sat'=>"18:00", 'sun'=>"18:00" )
								); // provide cutoff time for each shipping methods.

		$shipping_method=WC()->session->get( 'chosen_shipping_methods' );
		if(!empty($shipping_method))
		{
			$shipping_method=$shipping_method[0];
		}

		if(!empty($shipping_method)  && isset($adjustment[$shipping_method]) ) {
			$cutoff_time = !empty($adjustment[$shipping_method][$cur_day] ) ? $adjustment[$shipping_method][$cur_day]  : '';
			$cutoff_time 	= !empty( $cutoff_time ) ? $cutoff_time : '20:00';

			$cutoff_time = str_replace('.', ':', $cutoff_time);
			$cutoff=(strstr($cutoff_time, ':') ) ? explode( ':', $cutoff_time ) : array($cutoff_time, '');
		}
		return $cutoff;
	}
}